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/repositories/reference_repository.py

@@ -208,6 +208,15 @@ async def get_or_create_collection(
 
     existing = await _lookup()
     if existing is not None:
+        existing.name = name
+        # Only overwrite overview when the caller actually has one -- a
+        # movie's bare `belongs_to_collection` reference passes no overview
+        # at all, and must not blank out real data a full `ingest_collection`
+        # run already filled in (see get_or_create_person for the same
+        # partial-data guard).
+        if overview is not None:
+            existing.overview = overview
+        await session.flush()
         return existing
 
     async def _create() -> Collection: