fix(search): ts_stat 改单参(text),避免 'a' mask 静默 0 行

zhparser 不标 A 权重(也不标 B/C/D),传 'a' mask 给 ts_stat(text, weights) 会过滤掉所有词
但不报错,静默 0 行。改成 ts_stat(text) 单参(等价 mask='abcd',聚合所有权重)。

修:
- 0010 迁移里 refresh_search_keywords() 改用单参 ts_stat
- 0010 迁移 downgrade 部分同步修
- 0009 迁移 refresh_search_keywords() 同步修
- services/search.py _fallback_keywords 改用 chinese_zh + 单参 ts_stat

backend/alembic/versions/0009_search_suggestions.py

@@ -218,23 +218,19 @@ def upgrade() -> None:
                     FROM generate_series(1, length(word)) AS n
                 )
             FROM ts_stat(
-                'simple',
-                (
-                    SELECT to_tsvector(
-                        'simple',
-                        coalesce(title_zh, '') || ' ' ||
-                        coalesce(body_zh_text, '') || ' ' ||
-                        coalesce(commentary, '') || ' ' ||
-                        coalesce(commentary_meituan, '')
-                    )
-                    FROM articles
-                    WHERE title_zh IS NOT NULL
-                       OR body_zh_text IS NOT NULL
-                       OR commentary IS NOT NULL
-                       OR commentary_meituan IS NOT NULL
+                $$SELECT to_tsvector('simple',
+                    coalesce(title_zh, '') || ' ' ||
+                    coalesce(body_zh_text, '') || ' ' ||
+                    coalesce(commentary, '') || ' ' ||
+                    coalesce(commentary_meituan, '')
                 )
-            )
-            WHERE length(word) >= 2;  -- 过滤单字噪音(中文标点/单字停用词)
+                FROM articles
+                WHERE title_zh IS NOT NULL
+                   OR body_zh_text IS NOT NULL
+                   OR commentary IS NOT NULL
+                   OR commentary_meituan IS NOT NULL$$
+            ) AS s
+            WHERE length(s.word) >= 2;  -- 过滤单字噪音(中文标点/单字停用词)
 
         END;
         $$ LANGUAGE plpgsql;