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/models/__init__.py

@@ -7,6 +7,8 @@ from app.models.article import Article  # noqa: F401
 from app.models.article_read import ArticleRead  # noqa: F401
 from app.models.bookmark import Bookmark  # noqa: F401
 from app.models.llm_setting import LlmSetting  # noqa: F401
+from app.models.search_keyword import SearchKeyword  # noqa: F401
+from app.models.search_title_suggestion import SearchTitleSuggestion  # noqa: F401
 from app.models.source import Source, SourceKind  # noqa: F401
 from app.models.subscription import Subscription  # noqa: F401
 from app.models.user import User, UserRole  # noqa: F401
@@ -17,6 +19,8 @@ __all__ = [
     "ArticleRead",
     "Bookmark",
     "LlmSetting",
+    "SearchKeyword",
+    "SearchTitleSuggestion",
     "Source",
     "SourceKind",
     "Subscription",