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]>

app/providers/base.py

@@ -49,6 +49,11 @@ class MetadataProvider(ABC):
     async def get_person(self, external_id: str) -> dict[str, Any]:
         """Fetch raw person data, including external IDs and images."""
 
+    @abstractmethod
+    async def get_collection(self, external_id: str) -> dict[str, Any]:
+        """Fetch raw collection/franchise data: name, overview, poster/
+        backdrop, and its member movies (TMDb's "parts")."""
+
     @abstractmethod
     async def search_movie(self, query: str) -> list[dict[str, Any]]:
         """Search this provider's catalog for movies matching `query`. Used