Add collection/franchise ingestion

Collections were bare stubs (name only) lazily created the moment a
movie's belongs_to_collection reference was seen. Adds a real
ingest_collection pipeline: TMDb's /collection/{id} for overview and
poster/backdrop artwork, plus ordered CollectionMovie membership --
linking only movies already ingested into Aurora, the same
never-fabricate-a-stub rule movie recommendations already follow.

get_or_create_collection now refreshes name/overview on an existing
stub instead of only creating, guarded so a bare movie-stub call
(overview=None) never clobbers real data a full ingestion already
filled in.

New POST /api/v1/admin/ingest/collection endpoint and
ingest_collection_task; GET /api/v1/collections/{id} needed no changes
since its read path (collection_service.py) already assembled the full
detail shape.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

tests/integration/test_series_ingestion.py

@@ -39,6 +39,9 @@ class FakeTMDbSeriesProvider(MetadataProvider):
     async def get_person(self, external_id: str) -> dict[str, Any]:
         raise NotImplementedError
 
+    async def get_collection(self, external_id: str) -> dict[str, Any]:
+        raise NotImplementedError
+
     async def search_movie(self, query: str) -> list[dict[str, Any]]:
         raise NotImplementedError