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]>

tests/unit/test_normalizer.py

@@ -145,6 +145,21 @@ def test_normalize_movie_maps_artwork_and_videos():
     assert movie.videos[0].is_official is True
 
 
+def test_normalize_movie_marks_exactly_one_primary_poster_and_backdrop():
+    # is_primary picks out TMDb's top-level poster_path/backdrop_path (the
+    # image auto-downloaded for local hosting) from the full alternates
+    # list -- not just "the first one".
+    movie = normalize_movie(FAKE_MOVIE)
+    primary_posters = [a for a in movie.artwork if a.artwork_type == "poster" and a.is_primary]
+    primary_backdrops = [
+        a for a in movie.artwork if a.artwork_type == "backdrop" and a.is_primary
+    ]
+    assert len(primary_posters) == 1
+    assert primary_posters[0].provider_asset_id == "/poster.jpg"
+    assert len(primary_backdrops) == 1
+    assert primary_backdrops[0].provider_asset_id == "/backdrop.jpg"
+
+
 def test_normalize_movie_external_ids_never_include_tmdb_itself():
     # TMDb's own id is handled separately by the ingestion pipeline via
     # `normalized.external_id`, not folded into `external_ids` (section 3:
@@ -187,6 +202,9 @@ def test_normalize_person_profile_artwork_deduplicated():
     # not be inserted twice.
     paths = [a.provider_asset_id for a in person.artwork]
     assert paths.count("/keanu.jpg") == 1
+    # ...and since it wasn't inserted as a second row, the existing one must
+    # be the one marked primary instead -- not silently left unmarked.
+    assert next(a for a in person.artwork if a.provider_asset_id == "/keanu.jpg").is_primary
 
 
 FAKE_SERIES = {