feat(search): 智能搜索建议 - 固化候选词表 (search_keywords + search_title_suggestions)

后端:
- alembic 0009: 两张固化表 + GIN prefix_keys 索引 + articles trigger
- /api/v1/search/suggestions: 混合 A(高频词 ts_stat) + B(真实标题) + 冷启动 fallback
- worker 每日 03:00 + 启动时刷新 search_keywords
- 顺便填 commit 11 TODO: articles.title_zh_tsv + GIN 索引(未来 FTS 基础)

前端:
- NInput -> NAutoComplete + debounce 250ms
- 选标题 -> 跳详情;选关键词 -> 填入 + 触发搜索
- AbortController 防 race condition

性能: prefix_keys @> ARRAY[prefix] 走 GIN 亚毫秒,100w 行也稳

backend/app/main.py

@@ -16,7 +16,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 from starlette.exceptions import HTTPException as StarletteHTTPException
 
-from app.api import admin, admin_llm, articles, auth, bookmarks, ingest, me, sources, subscriptions
+from app.api import admin, admin_llm, articles, auth, bookmarks, ingest, me, search, sources, subscriptions
 from app.config import settings
 from app.database import engine
 from app.redis_client import close_redis, get_redis
@@ -100,6 +100,7 @@ app.include_router(sources.router, prefix=API_PREFIX)
 app.include_router(bookmarks.router, prefix=API_PREFIX)
 app.include_router(subscriptions.router, prefix=API_PREFIX)
 app.include_router(ingest.router, prefix=API_PREFIX)
+app.include_router(search.router, prefix=API_PREFIX)
 app.include_router(admin.router, prefix=API_PREFIX)
 app.include_router(admin_llm.router, prefix=API_PREFIX)