refactor(search): 只展示 keyword 续接词,去掉 titles 段

产品决定:搜索建议只展示 ts_stat 高频词续接(如'美'→美国/美军/美国政府),
不要真实文章 id 提示(用户认为这种'文章#566871'是噪音,没连续性)。

改动:
- SearchSuggestionsResponse 去 title,只剩 query + keywords
- SearchService 只查 search_keywords,fallback 路径也只针对 keywords
- Feed.vue: 删掉 suggestTitles 状态 + SuggestTitleOption 类型联合,
  renderSuggestion 简化成 '词' 标签 + 词文本 + 右侧 weight 数字
- 0011 迁移: 删 search_title_suggestions 表 + 3 索引 + trigger + 函数
  (trigger 在每篇文章 INSERT/UPDATE 都会跑,删了能省掉无用性能损耗)
- 删除: app/models/search_title_suggestion.py + backfill_search_suggestions.py
  替换成: app/scripts/refresh_search_keywords.py(只跑一次词频刷新)

backend/app/models/__init__.py

@@ -8,7 +8,6 @@ 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
@@ -20,7 +19,6 @@ __all__ = [
     "Bookmark",
     "LlmSetting",
     "SearchKeyword",
-    "SearchTitleSuggestion",
     "Source",
     "SourceKind",
     "Subscription",