feat(posts): support short titles for resource cards

Add optional post-level and localized title fields, and use a new
postTitleText helper for Resource.title so card/list surfaces prefer short
backend-provided titles instead of full body text. When title is missing,
fallback to the first non-empty body line, then filename, then post id.

Document the backend handoff in docs/posts-title-api.md alongside the other
backend task docs.

docs/posts-title-api.md

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+# Posts title fields for list/card surfaces
+
+Front-end list/card surfaces such as **热门资料**, **官方推荐**, and **全部资料** should display a short title, not the full Telegram/body text.
+
+## Affected endpoints
+
+Any endpoint returning `Post` items should include a short title when available:
+
+- `GET /api/posts`
+- `GET /api/posts/search`
+- `GET /api/posts/recommended`
+- `GET /api/posts/:id`
+
+## Recommended response shape
+
+```jsonc
+{
+  "id": "string",
+  "title": "ARK 2026 共识加速计划", // optional global fallback title
+  "text": "完整正文 / Telegram-style body text...",
+  "localizations": {
+    "zh": {
+      "title": "ARK 2026 共识加速计划",
+      "text": "完整中文正文...",
+    },
+    "en": {
+      "title": "ARK 2026 Consensus Acceleration Plan",
+      "text": "Full English body...",
+    },
+  },
+}
+```
+
+## Front-end fallback order
+
+For `Resource.title`, front-end reads:
+
+1. `localizations[currentLang].title`
+2. `post.title`
+3. first non-empty line of localized/full `text`
+4. first attachment filename
+5. `post.id`
+
+So backend can roll this out gradually: old posts without `title` still render, but long body text will be reduced to its first line.
+
+## Requirement
+
+Do **not** put an entire body paragraph into `title`. `title` should be concise enough for a two-line card/list title.
+
+Examples:
+
+| Good title                           | Bad title                                                 |
+| ------------------------------------ | --------------------------------------------------------- |
+| `ARK 2026「共识加速计划」邀请王霸榜` | Full event body with links, schedule, rules, and hashtags |
+| `ARK 主网核心合约地址（BSC链）`      | Full contract explainer paragraph                         |
+| `ARK灵魂五问完整视频`                | Full video caption text                                   |