fix(database): add missing search_official_title method to BangumiDatabase

BangumiDatabase.search_official_title() was called in
notification/manager.py (_get_poster) but never implemented,
causing a RenameThread crash:

  'BangumiDatabase' object has no attribute 'search_official_title'

Add the method as a simple exact-match lookup on Bangumi.official_title,
consistent with the existing search_id / search_rss pattern.

backend/src/module/database/bangumi.py

@@ -390,6 +390,10 @@ class BangumiDatabase:
         logger.debug("[Database] Find bangumi id: %s.", _id)
         return bangumi
 
+    def search_official_title(self, official_title: str) -> Optional[Bangumi]:
+        statement = select(Bangumi).where(Bangumi.official_title == official_title)
+        return self.session.execute(statement).scalar_one_or_none()
+
     def search_ids(self, ids: list[int]) -> list[Bangumi]:
         """Batch lookup multiple bangumi by their IDs."""
         if not ids: