diff --git a/.env.example b/.env.example
new file mode 100644
index 0000000..de0125e
--- /dev/null
+++ b/.env.example
@@ -0,0 +1,34 @@
+# Arkie Library Backend environment example
+# Copy to .env for your own reference, but note: the Go app does not auto-load .env.
+# Export these values in your shell, Docker, process manager, or deployment system.
+
+# HTTP
+HTTP_ADDR=:8080
+CORS_ORIGINS=http://localhost:5173,http://localhost:3000
+
+# PostgreSQL
+DATABASE_URL=postgres://ark:ark@localhost:5432/arkdb?sslmode=disable
+
+# Auth
+# Change this in production. Use a long random string.
+JWT_SECRET=dev-insecure-change-me
+
+# Wallet auth schema DDL at startup.
+# true is convenient for dev; false is safer for public/read-only DB users after migrations are applied.
+RUN_WALLET_AUTH_SCHEMA=true
+
+# Local uploads
+UPLOAD_DIR=./uploads
+UPLOAD_MULTIPART_MEM_MB=64
+
+# Optional first admin seed. Only creates an admin when the admins table is empty.
+SEED_ADMIN=false
+ADMIN_EMAIL=admin@ark.local
+ADMIN_PASSWORD=admin123
+
+# Optional S3 uploads. Leave S3_BUCKET empty to keep uploads local.
+S3_BUCKET=
+AWS_REGION=ap-southeast-1
+S3_UPLOAD_PREFIX=uploads
+# Optional CDN/website base URL, e.g. https://cdn.example.com
+S3_PUBLIC_BASE_URL=
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..647fd1b
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,81 @@
+# AI Agent Instructions
+
+This file is first-stop context for AI coding agents working in `Arkie-Library-Backend`.
+
+## Repository identity
+
+- Project: Arkie Library Backend / ARK database API.
+- Go module: `github.com/arkie/ark-database`.
+- Stack: Go, chi router, PostgreSQL/pgx, JWT, bcrypt, optional AWS S3 uploads, Ethereum wallet signature verification.
+- Frontend repo expects backend API at `/api` and uploaded assets under `/uploads`.
+
+## Branch and safety rules
+
+- Current branch observed during initialization: `terry-staging`.
+- Do not commit or push unless Terry explicitly asks.
+- Before branch-changing, pulling, rebasing, or destructive operations, run:
+
+```bash
+git status --short --branch
+```
+
+- Do not commit secrets. `.env.example` is safe; real `.env` files should stay local/deployment-only.
+
+## Required checks
+
+Before proposing deploy/push, run:
+
+```bash
+gofmt -w .
+go test ./...
+```
+
+Optional extra check:
+
+```bash
+go vet ./...
+```
+
+## Architecture quick map
+
+- `cmd/server/main.go`: starts everything. Loads config, connects DB, ensures upload dir, optionally seeds admin, sets up S3, registers routes.
+- `internal/config/config.go`: all environment variables and defaults.
+- `internal/db/db.go`: creates and pings a `pgxpool.Pool`.
+- `internal/auth/`: JWT helpers. Admin tokens use issuer `ark-admin`; wallet user tokens use issuer `ark-user`.
+- `internal/handlers/public.go`: public category/resource/search/metric endpoints.
+- `internal/handlers/admin.go`: admin login, dashboard, resource CRUD, tags.
+- `internal/handlers/wallet_auth.go`: wallet nonce, signature verification, wallet profile.
+- `internal/handlers/upload.go`: admin multipart upload to local disk or S3.
+- `migrations/`: SQL schema files; there is no Go migration runner yet.
+
+## Important implementation patterns
+
+- HTTP handlers are plain `net/http` functions using chi route params.
+- DB access comes from request context via `handlers.WithPool` and `poolFrom(r)`.
+- JSON responses use `writeJSON` in `internal/handlers/util.go`.
+- JSON request bodies use `jsonDecode`, limited to 1 MiB.
+- Resource IDs are UUIDs (`github.com/google/uuid`).
+- Tags are stored in `tags` + `resource_tags`; new tag slugs are deterministic hashes from `tagSlug`.
+- Public resource listing only returns `resources.status = 'published'` and `is_public = TRUE`.
+- Admin routes require `Authorization: Bearer <admin-jwt>` from `/api/admin/login`.
+- Wallet routes produce normal user JWTs, not admin JWTs.
+
+## Database/migration notes
+
+- `migrations/001_init.sql` is designed for a fresh database and is not fully idempotent.
+- `migrations/002_wallet_auth.sql` is idempotent.
+- `RUN_WALLET_AUTH_SCHEMA` defaults to true and lets startup create the wallet nonce table. In production with read-only/public DB users, apply migration separately and set it to false.
+
+## Deployment/runtime notes
+
+- Dockerfile builds a static Go binary in a `golang:1.24-alpine` builder and runs it on Alpine.
+- `UPLOAD_DIR` defaults to `./uploads`; Docker sets it to `/app/uploads`.
+- When `S3_BUCKET` is set and AWS config loads, uploads go to S3. Otherwise uploads stay local.
+- `JWT_SECRET` default is insecure and must be changed outside development.
+
+## Agent behavior preferences
+
+- Answer Terry concisely in Chinese unless code/docs require English.
+- Prefer small direct changes over broad rewrites.
+- Keep this README/AGENTS/docs updated when discovering non-obvious backend behavior.
+- Search project memory before making decisions about workflow, deployment, or conventions.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..26e2777
--- /dev/null
+++ b/README.md
@@ -0,0 +1,131 @@
+# Arkie Library Backend
+
+Go backend for the ARK Library / ARK Database website.
+
+If Go is new for you: start with **[docs/GO_FOR_BEGINNERS.md](docs/GO_FOR_BEGINNERS.md)**, then read **[docs/ARCHITECTURE.md](docs/ARCHITECTURE.md)**.
+
+## What this service does
+
+- Serves public library data under `/api`.
+- Serves uploaded files under `/uploads`.
+- Provides admin login and admin CRUD APIs for resources.
+- Supports wallet sign-in with Ethereum personal-sign messages.
+- Stores data in PostgreSQL.
+- Stores uploads locally by default, or in S3 when configured.
+
+## Tech stack
+
+- Go module: `github.com/arkie/ark-database`
+- HTTP router: `github.com/go-chi/chi/v5`
+- Database: PostgreSQL via `github.com/jackc/pgx/v5/pgxpool`
+- Auth: JWT via `github.com/golang-jwt/jwt/v5`
+- Password hashing: bcrypt
+- Wallet signature verification: `github.com/ethereum/go-ethereum`
+- Optional uploads: AWS S3 SDK v2
+
+## Folder map
+
+```text
+cmd/server/main.go          # application entry point: config, DB, router, endpoints
+internal/config/config.go   # environment variables and default values
+internal/db/db.go           # PostgreSQL connection pool
+internal/auth/              # JWT signing/parsing for admin and wallet users
+internal/handlers/          # HTTP handlers: public, admin, upload, wallet auth
+internal/seed/seed.go       # optional initial admin user
+migrations/                 # SQL schema files, apply manually
+static/                     # bundled static assets copied into uploads
+```
+
+## Quick local run
+
+### 1) Start PostgreSQL
+
+Example with Docker:
+
+```bash
+docker run --name ark-postgres \
+  -e POSTGRES_USER=ark \
+  -e POSTGRES_PASSWORD=ark \
+  -e POSTGRES_DB=arkdb \
+  -p 5432:5432 \
+  -d postgres:16
+```
+
+### 2) Apply schema once
+
+`migrations/001_init.sql` is for a fresh empty database. Do not run it twice on the same DB unless you reset the DB first.
+
+```bash
+cd Arkie-Library-Backend
+export DATABASE_URL='postgres://ark:ark@localhost:5432/arkdb?sslmode=disable'
+psql "$DATABASE_URL" -f migrations/001_init.sql
+```
+
+Wallet nonce schema is normally auto-created at app startup because `RUN_WALLET_AUTH_SCHEMA` defaults to true. If you want to apply it manually and disable startup DDL:
+
+```bash
+psql "$DATABASE_URL" -f migrations/002_wallet_auth.sql
+export RUN_WALLET_AUTH_SCHEMA=false
+```
+
+### 3) Configure env
+
+Copy the example and edit values as needed:
+
+```bash
+cp .env.example .env
+```
+
+This project does not auto-load `.env`; either export variables in your shell or run with a dotenv helper.
+
+### 4) Run the server
+
+```bash
+go run ./cmd/server
+```
+
+Check health:
+
+```bash
+curl http://localhost:8080/healthz
+```
+
+## Useful commands
+
+```bash
+# Format all Go files
+gofmt -w .
+
+# Compile and run all package tests
+go test ./...
+
+# Run the backend
+go run ./cmd/server
+
+# Build a local binary
+go build -o ./bin/server ./cmd/server
+```
+
+## Environment variables
+
+See `.env.example` for all variables. Important ones:
+
+| Variable | Default | Purpose |
+| --- | --- | --- |
+| `HTTP_ADDR` | `:8080` | Listen address |
+| `DATABASE_URL` | `postgres://ark:ark@localhost:5432/arkdb?sslmode=disable` | PostgreSQL connection |
+| `JWT_SECRET` | `dev-insecure-change-me` | JWT signing key; change in production |
+| `UPLOAD_DIR` | `./uploads` | Local upload directory |
+| `CORS_ORIGINS` | empty | Comma-separated allowed browser origins |
+| `SEED_ADMIN` | false | Set `true`/`1` to create first admin if none exists |
+| `ADMIN_EMAIL` | `admin@ark.local` | Seed admin email |
+| `ADMIN_PASSWORD` | `admin123` | Seed admin password |
+| `S3_BUCKET` | empty | Enables S3 uploads when set |
+
+## API docs
+
+See **[docs/API.md](docs/API.md)**.
+
+## Agent notes
+
+AI/coding agent instructions are in **[AGENTS.md](AGENTS.md)**.
diff --git a/docs/API.md b/docs/API.md
new file mode 100644
index 0000000..e2a839a
--- /dev/null
+++ b/docs/API.md
@@ -0,0 +1,360 @@
+# API Reference
+
+Base URL in local development: `http://localhost:8080`.
+
+Frontend usually calls paths like `/api/resources`; uploaded files are available under `/uploads/...`.
+
+## Response style
+
+Most responses are JSON. Errors use plain text via `http.Error`, for example `unauthorized` or `bad json`.
+
+## Health
+
+### `GET /healthz`
+
+Returns plain text:
+
+```text
+ok
+```
+
+### `GET /health`
+
+Same as `/healthz`.
+
+## Public categories/resources
+
+### `GET /api/categories`
+
+Query/header language behavior:
+
+- `?lang=zh-CN` returns Simplified Chinese name when available.
+- `?lang=en` returns English name when available.
+- otherwise Traditional Chinese is preferred.
+
+Response:
+
+```json
+[
+  {
+    "id": 1,
+    "slug": "project-ppt",
+    "name": "項目資料（PPT）",
+    "description": "ARK 項目介紹、簡報與對外展示資料",
+    "iconKey": "folder",
+    "sortOrder": 1,
+    "updatedAt": "2026-01-01T00:00:00Z"
+  }
+]
+```
+
+### `GET /api/resources`
+
+Returns paginated public resources.
+
+Query params:
+
+| Param | Meaning |
+| --- | --- |
+| `page` | page number, default 1 |
+| `limit` | page size, default 20, max 100 |
+| `q` | search title/description/tag name |
+| `type` | resource type; `all` means no filter |
+| `language` | exact resource language filter |
+| `category` | category slug |
+| `tag` | tag slug or tag name, case-insensitive |
+| `sort` | `latest` default, `published`, `recommended`, `popular` |
+| `lang` | category display language, e.g. `en`, `zh-CN` |
+
+Response:
+
+```json
+{
+  "items": [
+    {
+      "id": "uuid",
+      "title": "ARK 項目介紹簡報（示例）",
+      "description": "適合線下宣講與新人培訓。",
+      "type": "ppt",
+      "language": "zh-TW",
+      "categoryId": 1,
+      "categorySlug": "project-ppt",
+      "categoryName": "項目資料（PPT）",
+      "coverImage": "/uploads/placeholder-cover.svg",
+      "fileUrl": "/uploads/placeholder-cover.svg",
+      "isDownloadable": true,
+      "isRecommended": true,
+      "publishedAt": "2026-01-01T00:00:00Z",
+      "updatedAt": "2026-01-01T00:00:00Z",
+      "tags": ["官方推薦"]
+    }
+  ],
+  "page": 1,
+  "limit": 20,
+  "total": 1
+}
+```
+
+### `GET /api/resources/recommended?limit=12`
+
+Returns recommended resources:
+
+```json
+{ "items": [] }
+```
+
+Max limit is 50.
+
+### `GET /api/resources/latest?limit=12`
+
+Returns latest resources:
+
+```json
+{ "items": [] }
+```
+
+### `GET /api/resources/popular?limit=12`
+
+Returns resources ordered by download + favorite + share count:
+
+```json
+{ "items": [] }
+```
+
+### `GET /api/resources/{id}`
+
+Returns one published public resource. `{id}` must be a UUID.
+
+### `GET /api/resources/{id}/related`
+
+Returns up to 8 resources from the same category:
+
+```json
+{ "items": [] }
+```
+
+## Public activity endpoints
+
+### `POST /api/search-log`
+
+Request:
+
+```json
+{ "query": "ark" }
+```
+
+Response:
+
+```json
+{ "ok": true }
+```
+
+### `POST /api/resources/{id}/view`
+
+Increments view count.
+
+Response:
+
+```json
+{ "ok": true }
+```
+
+### `POST /api/resources/{id}/download`
+
+Increments download count.
+
+### `POST /api/resources/{id}/share`
+
+Increments share count.
+
+### `POST /api/resources/{id}/favorite`
+
+Request:
+
+```json
+{ "add": true }
+```
+
+- `add: true` increments favorite count.
+- `add: false` decrements favorite count but not below zero.
+
+## Wallet auth
+
+Wallet auth is for normal users, not admin users.
+
+### `POST /api/auth/wallet/nonce`
+
+Request:
+
+```json
+{ "address": "0x0000000000000000000000000000000000000000" }
+```
+
+Response:
+
+```json
+{
+  "nonce": "hex-code",
+  "message": "ARK Database — wallet sign-in\n\nWallet: 0x...\nOne-time code: ..."
+}
+```
+
+The frontend asks the wallet to sign `message`.
+
+### `POST /api/auth/wallet/verify`
+
+Request:
+
+```json
+{
+  "address": "0x0000000000000000000000000000000000000000",
+  "message": "same message from nonce endpoint",
+  "signature": "0x..."
+}
+```
+
+Response:
+
+```json
+{
+  "token": "jwt",
+  "wallet": "0xChecksumAddress"
+}
+```
+
+### `GET /api/auth/wallet/me`
+
+Requires header:
+
+```http
+Authorization: Bearer <wallet-jwt>
+```
+
+Response:
+
+```json
+{ "wallet": "0xChecksumAddress", "role": "user" }
+```
+
+## Admin auth
+
+### `POST /api/admin/login`
+
+Request:
+
+```json
+{ "email": "admin@ark.local", "password": "admin123" }
+```
+
+Response:
+
+```json
+{ "token": "admin-jwt" }
+```
+
+Use this token for admin endpoints:
+
+```http
+Authorization: Bearer <admin-jwt>
+```
+
+## Admin endpoints
+
+All endpoints below require admin JWT.
+
+### `GET /api/admin/dashboard`
+
+Response includes counts and hot resources:
+
+```json
+{
+  "totalResources": 10,
+  "published": 8,
+  "todayNew": 1,
+  "totalViews": 100,
+  "totalDownloads": 20,
+  "totalFavorites": 5,
+  "totalShares": 3,
+  "hotResources": []
+}
+```
+
+### `GET /api/admin/search-logs?limit=200`
+
+Max limit is 500.
+
+Response:
+
+```json
+{ "items": [{ "id": 1, "query": "ark", "createdAt": "2026-01-01T00:00:00Z" }] }
+```
+
+### `GET /api/admin/resources?page=1&limit=20`
+
+Lists all resources, including drafts/private resources.
+
+### `GET /api/admin/resources/{id}`
+
+Gets one resource by UUID.
+
+### `POST /api/admin/resources`
+
+Creates a resource.
+
+Example request body:
+
+```json
+{
+  "title": "Example",
+  "description": "Short text",
+  "type": "ppt",
+  "language": "zh-TW",
+  "categoryId": 1,
+  "coverImage": "/uploads/example.svg",
+  "fileUrl": "/uploads/example.pdf",
+  "previewUrl": "",
+  "externalUrl": "",
+  "bodyText": "",
+  "badgeLabel": "新人必看",
+  "isPublic": true,
+  "isDownloadable": true,
+  "isRecommended": false,
+  "sortOrder": 0,
+  "status": "draft",
+  "tags": ["教程"]
+}
+```
+
+### `PUT /api/admin/resources/{id}`
+
+Updates a resource. Body shape is the same as create.
+
+### `DELETE /api/admin/resources/{id}`
+
+Deletes a resource.
+
+Response:
+
+```json
+{ "ok": true }
+```
+
+### `POST /api/admin/upload`
+
+Multipart upload with field name `file`.
+
+Response for local storage:
+
+```json
+{ "url": "/uploads/generated-name.ext", "filename": "generated-name.ext", "storage": "local" }
+```
+
+Response for S3:
+
+```json
+{ "url": "https://...", "filename": "generated-name.ext", "storage": "s3" }
+```
+
+### `GET /api/admin/categories`
+
+Same handler as public categories; returns visible categories.
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
new file mode 100644
index 0000000..780c9d9
--- /dev/null
+++ b/docs/ARCHITECTURE.md
@@ -0,0 +1,180 @@
+# Backend Architecture
+
+This document explains the backend in beginner-friendly language.
+
+## Big picture
+
+```text
+Browser / Frontend
+        |
+        | HTTP JSON requests
+        v
+cmd/server/main.go
+        |
+        +-- config.Load() reads environment variables
+        +-- db.Connect() creates PostgreSQL pool
+        +-- chi router maps URLs to handler functions
+        +-- handlers read/write DB and return JSON
+        |
+        v
+PostgreSQL + local uploads or S3
+```
+
+The backend is one Go HTTP server. There is no framework magic: `main.go` wires everything manually.
+
+## Request flow example
+
+Example: frontend calls `GET /api/resources?limit=20`.
+
+1. `main.go` has this route:
+   ```go
+   r.Get("/resources", handlers.ListResources)
+   ```
+2. The `/api` router has middleware:
+   ```go
+   r.Use(func(next http.Handler) http.Handler {
+       return handlers.WithPool(next, pool)
+   })
+   ```
+   This places the PostgreSQL pool into the request context.
+3. `handlers.ListResources` calls `poolFrom(r)` to get the DB pool.
+4. It reads query string filters (`q`, `type`, `language`, `category`, `sort`, `tag`).
+5. It builds SQL, queries PostgreSQL, scans rows into `ResourceDTO`.
+6. It returns JSON using `writeJSON`.
+
+## Why `internal/`?
+
+In Go, a folder named `internal` can only be imported by code inside the same module tree. This prevents other modules from depending on private implementation packages.
+
+This project uses:
+
+```text
+internal/config   private config package
+internal/db       private DB package
+internal/auth     private JWT package
+internal/handlers private HTTP handler package
+internal/seed     private seed package
+```
+
+## Main packages
+
+### `cmd/server`
+
+`cmd/server/main.go` is the executable entry point.
+
+Responsibilities:
+
+- Load environment config.
+- Connect to PostgreSQL.
+- Ensure wallet nonce table when enabled.
+- Create upload directory and placeholder file.
+- Optionally seed the first admin.
+- Configure optional S3 upload client.
+- Register all HTTP middleware and routes.
+- Start `http.ListenAndServe`.
+
+### `internal/config`
+
+`config.Load()` reads env vars and fills defaults. It returns a `Config` struct.
+
+Important pattern:
+
+```go
+addr := os.Getenv("HTTP_ADDR")
+if addr == "" {
+    addr = ":8080"
+}
+```
+
+This means env vars are optional in local development.
+
+### `internal/db`
+
+`db.Connect(ctx, databaseURL)` parses `DATABASE_URL`, creates a pgx connection pool, then pings the DB.
+
+The app shares this pool for all requests.
+
+### `internal/auth`
+
+Two JWT token types exist:
+
+- Admin token: issuer `ark-admin`, includes `admin_id` and email.
+- Wallet user token: issuer `ark-user`, includes wallet address and role `user`.
+
+Do not mix these. Admin middleware only accepts admin tokens.
+
+### `internal/handlers`
+
+Handlers are grouped by topic:
+
+- `public.go`: public category/resource list/detail, search logs, counters.
+- `admin.go`: admin login/dashboard/resource CRUD/tag replacement.
+- `wallet_auth.go`: wallet nonce and signature verification.
+- `upload.go`: admin file upload.
+- `middleware.go`: admin auth middleware.
+- `context.go`: stores DB pool in request context.
+- `util.go`: shared JSON helpers.
+
+## Database model summary
+
+Main tables from `migrations/001_init.sql`:
+
+- `admins`: admin email + bcrypt password hash.
+- `categories`: resource categories with multilingual names.
+- `resources`: library items; can be ppt/image/video/link/etc.
+- `tags`: reusable tag names.
+- `resource_tags`: many-to-many join table.
+- `search_logs`: saved public search terms.
+
+Wallet auth table from migration/startup DDL:
+
+- `wallet_auth_nonces`: one temporary nonce per wallet address.
+
+## Route groups
+
+### Public routes
+
+These do not require auth:
+
+- health: `/healthz`, `/health`
+- categories/resources under `/api`
+- wallet auth nonce/verify/me (me requires wallet JWT)
+- resource view/download/share/favorite counters
+
+### Admin routes
+
+`POST /api/admin/login` is public because it returns the admin token.
+
+Everything in this group requires admin JWT:
+
+- dashboard
+- search logs
+- resource list/get/create/update/delete
+- upload
+- admin categories list
+
+## Upload behavior
+
+Admin upload endpoint: `POST /api/admin/upload` with multipart field `file`.
+
+If S3 is configured:
+
+```text
+S3_BUCKET non-empty + AWS config loads => upload to S3
+```
+
+Otherwise:
+
+```text
+save file to UPLOAD_DIR and return /uploads/<filename>
+```
+
+The max upload size is 512 MiB (`uploadMaxBytes`).
+
+## Things to be careful with
+
+- `migrations/001_init.sql` is not safe to run repeatedly on the same DB.
+- `JWT_SECRET` default is for dev only.
+- `RUN_WALLET_AUTH_SCHEMA=true` runs DDL at startup. Use false if the runtime DB user should not create tables.
+- The app does not auto-load `.env`.
+- Public resources are filtered to published + public; admin list sees all resources.
diff --git a/docs/GO_FOR_BEGINNERS.md b/docs/GO_FOR_BEGINNERS.md
new file mode 100644
index 0000000..05bdb66
--- /dev/null
+++ b/docs/GO_FOR_BEGINNERS.md
@@ -0,0 +1,281 @@
+# Go Beginner Guide for This Backend
+
+This is not a full Go course. It explains only the Go ideas you need to read this project.
+
+## 1. How a Go project starts
+
+This repo has a `go.mod` file:
+
+```go
+module github.com/arkie/ark-database
+```
+
+That module name is used in imports:
+
+```go
+import "github.com/arkie/ark-database/internal/config"
+```
+
+The executable starts at:
+
+```text
+cmd/server/main.go
+```
+
+In Go, a runnable program has:
+
+```go
+package main
+
+func main() {
+    // program starts here
+}
+```
+
+## 2. Packages
+
+Every `.go` file begins with a package name:
+
+```go
+package handlers
+```
+
+Files in the same folder usually share the same package and can call each other directly.
+
+Example: files in `internal/handlers/` all say `package handlers`, so `public.go` can call `writeJSON` from `util.go`.
+
+## 3. Imports
+
+Go imports are explicit:
+
+```go
+import (
+    "net/http"
+    "github.com/go-chi/chi/v5"
+)
+```
+
+If an import is unused, Go compilation fails. Run `gofmt -w .` after edits.
+
+## 4. Structs
+
+A struct is an object/data shape.
+
+Example from `internal/config/config.go`:
+
+```go
+type Config struct {
+    Addr        string
+    DatabaseURL string
+    JWTSecret   string
+}
+```
+
+JSON field names are controlled by tags:
+
+```go
+type ResourceDTO struct {
+    CategoryID int `json:"categoryId"`
+}
+```
+
+The Go field is `CategoryID`, but JSON output is `categoryId`.
+
+## 5. Functions with errors
+
+Go commonly returns `(value, error)`:
+
+```go
+pool, err := db.Connect(ctx, cfg.DatabaseURL)
+if err != nil {
+    log.Fatal(err)
+}
+```
+
+Read this as:
+
+1. Try connecting to DB.
+2. If `err` is not nil, stop.
+3. Otherwise use `pool`.
+
+## 6. Short variable declaration `:=`
+
+This creates a new variable:
+
+```go
+cfg := config.Load()
+```
+
+This assigns to an existing variable:
+
+```go
+cfg = config.Load()
+```
+
+Most code here uses `:=` inside functions.
+
+## 7. HTTP handlers
+
+A Go HTTP handler usually looks like:
+
+```go
+func ListCategories(w http.ResponseWriter, r *http.Request) {
+    // w writes response
+    // r reads request
+}
+```
+
+- `w` = response writer.
+- `r` = request.
+
+Route registration in `main.go`:
+
+```go
+r.Get("/categories", handlers.ListCategories)
+```
+
+Because this is inside `r.Route("/api", ...)`, the full URL is:
+
+```text
+GET /api/categories
+```
+
+## 8. Middleware
+
+Middleware wraps a request before it reaches the final handler.
+
+Example in `main.go`:
+
+```go
+r.Use(middleware.Logger)
+```
+
+This logs requests.
+
+Admin auth is also middleware:
+
+```go
+r.Use(handlers.AdminAuth(cfg.JWTSecret))
+```
+
+If token is bad, it returns `401 unauthorized` before reaching the admin handler.
+
+## 9. Context
+
+Context carries request-scoped values and cancellation.
+
+This project stores the DB pool in request context:
+
+```go
+r.Use(func(next http.Handler) http.Handler {
+    return handlers.WithPool(next, pool)
+})
+```
+
+Handlers retrieve it:
+
+```go
+pool := poolFrom(r)
+```
+
+So most handlers do not receive the DB pool as a direct function parameter.
+
+## 10. Database queries
+
+This project uses pgx.
+
+One row:
+
+```go
+err := pool.QueryRow(ctx, `SELECT id FROM admins WHERE email = $1`, email).Scan(&id)
+```
+
+Many rows:
+
+```go
+rows, err := pool.Query(ctx, `SELECT id, title FROM resources`)
+defer rows.Close()
+for rows.Next() {
+    rows.Scan(&id, &title)
+}
+```
+
+PostgreSQL parameters use `$1`, `$2`, etc. This protects from SQL injection when used correctly.
+
+## 11. Pointers and nil
+
+You will see `*string` and `*time.Time`:
+
+```go
+var pubAt *time.Time
+```
+
+This allows SQL `NULL` to become Go `nil`.
+
+Code checks before using it:
+
+```go
+if pubAt != nil {
+    s := pubAt.UTC().Format(time.RFC3339)
+    dto.PublishedAt = &s
+}
+```
+
+## 12. `defer`
+
+`defer` runs later when the current function returns.
+
+Examples:
+
+```go
+defer pool.Close()
+defer rows.Close()
+defer r.Body.Close()
+```
+
+Use it for cleanup.
+
+## 13. JSON helpers in this project
+
+Write JSON response:
+
+```go
+writeJSON(w, map[string]any{"ok": true})
+```
+
+Read JSON request body:
+
+```go
+var req loginReq
+if err := jsonDecode(r, &req); err != nil {
+    http.Error(w, "bad json", http.StatusBadRequest)
+    return
+}
+```
+
+## 14. Common edit workflow
+
+After changing `.go` files:
+
+```bash
+gofmt -w .
+go test ./...
+```
+
+`go test ./...` also compiles all packages, even if there are no test files.
+
+## 15. Recommended reading order
+
+1. `README.md`
+2. `cmd/server/main.go`
+3. `internal/config/config.go`
+4. `internal/handlers/public.go`
+5. `internal/handlers/admin.go`
+6. `internal/handlers/wallet_auth.go`
+7. `migrations/001_init.sql`
+
+When you see a function you do not understand, search for its definition:
+
+```bash
+rg "func FunctionName" .
+```