Add artwork self-hosting: local-disk download worker + storage

Section 10: every image API responses reference has pointed straight
at TMDb's CDN since day one -- Aurora never actually downloaded or
served artwork itself, despite storing dimensions/checksum/local_path
columns for exactly that. Closes the gap:

- is_primary was declared on the Artwork model but never actually set
  anywhere -- every row defaulted to False. Now the normalizer marks
  the one image each provider designates as *the* image for an entity
  (TMDb's poster_path/backdrop_path/profile_path) as primary,
  distinct from the dozens of alternates also ingested, including
  correctly marking an existing row primary (not inserting a
  duplicate) when that image already appears in the alternates list.
- app/services/artwork/: an ArtworkStorage interface with a
  LocalDiskStorage implementation (checksum-named files, single-host
  deployment via a Docker volume shared between api and worker -- see
  docs/architecture.md for the S3/R2 upgrade path), and a download
  pipeline with content-type validation, a size cap, and path-traversal
  protection.
- After movie/series/person ingestion succeeds, tasks.py cascades one
  download_artwork_task per primary artwork id via arq's ctx['redis']
  (same fan-out pattern as the season cascade) -- fire-and-forget, not
  tracked in ingestion_jobs, since a failed image cache is a
  known-safe fallback to the provider URL, not a user-facing failure.
- app/main.py serves MEDIA_ROOT at MEDIA_URL_PREFIX; local_path is
  preferred over original_url once set (already-existing fallback
  logic in app/services/common.py:artwork_url, now actually exercised).

Also fixes a real interaction bug this surfaced: every ingestion
pipeline deleted and recreated all of an entity's Artwork rows on each
re-sync, which would have silently wiped local_path/checksum on every
re-ingestion, forcing a full re-download every time and defeating the
point of caching locally at all. app/repositories/polymorphic.py's new
sync_artwork upserts by provider_asset_id (TMDb's stable per-image
path) instead, preserving downloaded copies across re-sync, and
replaces three near-identical delete+recreate helpers that used to
live separately in movie_ingestion.py, series_ingestion.py, and
person_ingestion.py.

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

docker-compose.prod.yml

@@ -87,6 +87,9 @@ services:
       DATABASE_BEHIND_TRANSACTION_POOLER: "true"
       DATABASE_URL: postgresql+asyncpg://${POSTGRES_USER:-aurora}:${POSTGRES_PASSWORD}@pgbouncer:5432/${POSTGRES_DB:-aurora_core}
       REDIS_URL: redis://redis:6379/0
+      MEDIA_ROOT: /app/media
+    volumes:
+      - aurora_media_data:/app/media
     depends_on:
       pgbouncer:
         condition: service_healthy
@@ -119,6 +122,9 @@ services:
       DATABASE_BEHIND_TRANSACTION_POOLER: "true"
       DATABASE_URL: postgresql+asyncpg://${POSTGRES_USER:-aurora}:${POSTGRES_PASSWORD}@pgbouncer:5432/${POSTGRES_DB:-aurora_core}
       REDIS_URL: redis://redis:6379/0
+      MEDIA_ROOT: /app/media
+    volumes:
+      - aurora_media_data:/app/media
     depends_on:
       pgbouncer:
         condition: service_healthy
@@ -135,3 +141,4 @@ services:
 volumes:
   aurora_postgres_data:
   aurora_redis_data:
+  aurora_media_data: