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/workers/worker.py

@@ -15,6 +15,7 @@ from app.core.config import get_settings
 from app.core.logging import configure_logging, get_logger
 from app.workers.tasks import (
     download_artwork_task,
+    ingest_collection_task,
     ingest_movie_task,
     ingest_person_task,
     ingest_season_task,
@@ -39,6 +40,7 @@ class WorkerSettings:
         ingest_person_task,
         ingest_series_task,
         ingest_season_task,
+        ingest_collection_task,
         download_artwork_task,
     ]
     on_startup = startup