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/integrations/objectstore/base.py

@@ -0,0 +1,22 @@
+"""ObjectStore interface — pluggable binary storage behind the service layer.
+
+Implementations are S3-compatible (MinIO for self-host, any S3 otherwise).
+Methods are async wrappers so the service layer stays non-blocking even though
+the underlying SDK (boto3) is synchronous.
+"""
+
+from abc import ABC, abstractmethod
+
+
+class ObjectStore(ABC):
+    @abstractmethod
+    async def ensure_bucket(self) -> None: ...
+
+    @abstractmethod
+    async def put_object(self, *, key: str, data: bytes, content_type: str) -> None: ...
+
+    @abstractmethod
+    async def presigned_get_url(self, *, key: str) -> str: ...
+
+    @abstractmethod
+    async def delete_object(self, *, key: str) -> None: ...