refactor(BA-5837): rename user_domain_name -> get_user_domain_name

The lookup is a single-column fetch helper (not an entity getter), so
the explicit `get_` prefix matches sister DB-source helpers and reads
naturally at the call site. Update the only caller in
`AppConfigFragmentRepository.app_config` accordingly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/manager/repositories/app_config_fragment/db_source/db_source.py

@@ -104,17 +104,18 @@ async def get_by_id(self, id: uuid.UUID) -> AppConfigFragmentData | None:
             return row.to_data() if row is not None else None
 
     @app_config_fragment_db_source_resilience.apply()
-    async def user_domain_name(self, user_id: uuid.UUID) -> str | None:
+    async def get_user_domain_name(self, user_id: uuid.UUID) -> str | None:
         """Single-column lookup of a user's `domain_name`.
 
         Used by the cache layer to tag merged-view entries with their
         owning domain so domain-scoped fragment writes can target a
         bounded user set during invalidation.
         """
         async with self._db.begin_readonly_session() as db_sess:
-            return await db_sess.scalar(
+            domain_name: str | None = await db_sess.scalar(
                 sa.select(UserRow.domain_name).where(UserRow.uuid == user_id)
             )
+            return domain_name
 
     @app_config_fragment_db_source_resilience.apply()
     async def create(self, creator: Creator[AppConfigFragmentRow]) -> AppConfigFragmentData:

src/ai/backend/manager/repositories/app_config_fragment/repository.py

@@ -116,7 +116,7 @@ async def app_config(
 
         merged = await self._db_source.get_user_app_config(user_id, config_name)
         if self._cache_source is not None:
-            domain_name = await self._db_source.user_domain_name(user_id)
+            domain_name = await self._db_source.get_user_domain_name(user_id)
             await self._cache_source.set_merged_config(merged, domain_name=domain_name)
         return merged