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/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.audit import AuditEntry
 from app.models.auth import Session, UserToken
 from app.models.base import Base
 from app.models.event import Event
+from app.models.media import Media
 from app.models.person import Name, Person
 from app.models.place import Place, PlaceName
 from app.models.relationship import Relationship
@@ -28,4 +29,5 @@ __all__ = [
     "AuditEntry",
     "Session",
     "UserToken",
+    "Media",
 ]