Add media (object storage) and the background worker (Phase 1)

Media model + migration; an ObjectStore interface with an S3/MinIO (boto3) implementation behind the service layer. Upload (multipart) stores bytes in object storage + a metadata row (checksum, size, content-type, optional attach to person/event/source); list returns presigned URLs; delete is soft. Editor-gated, privacy-filtered, audited. 24 tests pass (object store faked).

Introduces the worker container (same image, 'python -m app.worker'): its first job is the scheduled 30-day soft-delete purge across tables + media object cleanup. Compose gains worker + S3 env on backend/worker; dev override builds the worker too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/api/v1/__init__.py

@@ -6,6 +6,7 @@ from app.api.v1 import (
     auth,
     citations,
     events,
+    media,
     persons,
     relationships,
     sources,
@@ -22,3 +23,4 @@ api_router.include_router(events.router)
 api_router.include_router(relationships.router)
 api_router.include_router(sources.router)
 api_router.include_router(citations.router)
+api_router.include_router(media.router)