docs: add backend onboarding guide

.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=