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",

backend/app/models/search_title_suggestion.py

@@ -1,43 +0,0 @@
-"""搜索建议 - 真实文章标题片段表(articles 写入 trigger 自动维护)。
-
-- 数据源:articles.title_zh(优先)/ articles.title(短新闻回退)
-- 用途:/api/v1/search/suggestions 返回"真实文章标题"建议(B 方案)
-- 维护:PG trigger(articles INSERT/UPDATE OF title_zh/title/published_at 触发)
-- 查询:prefix_keys @> ARRAY['美'] 走 GIN 索引,按 published_at DESC 排序
-"""
-from __future__ import annotations
-
-from datetime import datetime
-
-from sqlalchemy import BigInteger, DateTime, ForeignKey, String, func
-from sqlalchemy.dialects.postgresql import ARRAY, TEXT
-from sqlalchemy.orm import Mapped, mapped_column
-
-from app.database import Base
-
-
-class SearchTitleSuggestion(Base):
-    __tablename__ = "search_title_suggestions"
-
-    id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
-
-    article_id: Mapped[int] = mapped_column(
-        BigInteger,
-        ForeignKey("articles.id", ondelete="CASCADE"),
-        nullable=False,
-    )
-
-    # 该条用的是哪边的文本:'zh' (title_zh) / 'src' (title 短新闻回退)
-    title_lang: Mapped[str] = mapped_column(String(8), nullable=False, default="zh")
-
-    # 预计算前缀数组(从第 1 字到全词)
-    prefix_keys: Mapped[list[str]] = mapped_column(ARRAY(TEXT), nullable=False)
-
-    published_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
-
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), server_default=func.now(), nullable=False
-    )
-
-    def __repr__(self) -> str:
-        return f"<SearchTitleSuggestion article_id={self.article_id} lang={self.title_lang}>"