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]>

app/services/ingestion/person_ingestion.py

@@ -10,14 +10,13 @@ from __future__ import annotations
 
 import uuid
 
-from sqlalchemy import delete
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.logging import get_logger
-from app.models.artwork import Artwork
-from app.models.enums import ArtworkType, MatchConfidence
+from app.models.enums import MatchConfidence
 from app.models.person import Person
 from app.providers.base import MetadataProvider
+from app.repositories import polymorphic
 from app.repositories import reference_repository as refs
 from app.repositories.provider_repository import get_or_create_provider, upsert_provider_record
 from app.services.metadata.matcher import match_entity
@@ -64,24 +63,13 @@ async def ingest_person(
 
     await session.flush()
 
-    await session.execute(
-        delete(Artwork).where(Artwork.entity_type == "person", Artwork.entity_id == person.id)
+    await polymorphic.sync_artwork(
+        session,
+        entity_type="person",
+        entity_id=person.id,
+        provider_id=tmdb_provider.id,
+        artwork_list=normalized.artwork,
     )
-    for art in normalized.artwork:
-        session.add(
-            Artwork(
-                entity_type="person",
-                entity_id=person.id,
-                artwork_type=ArtworkType.PROFILE,
-                width=art.width,
-                height=art.height,
-                aspect_ratio=art.aspect_ratio,
-                provider_id=tmdb_provider.id,
-                provider_asset_id=art.provider_asset_id,
-                original_url=art.url,
-                vote_average=art.vote_average,
-            )
-        )
 
     await refs.link_external_id(
         session,