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/core/config.py

@@ -35,6 +35,18 @@ class Settings(BaseSettings):
     # Base URL used to build links in outbound email.
     app_base_url: str = "http://localhost"
 
+    # --- Object storage (S3-compatible / MinIO) ---
+    s3_endpoint_url: str = "http://minio:9000"
+    s3_bucket: str = "provenance"
+    s3_access_key: str = "provenance"
+    s3_secret_key: str = "change-me-too"
+    s3_region: str = "us-east-1"
+    s3_presign_ttl: int = 3600  # seconds
+
+    # --- Worker ---
+    purge_interval_seconds: int = 3600  # how often to run the soft-delete purge
+    purge_after_days: int = 30  # soft-deleted rows older than this are purged
+
     # --- Email (SMTP) ---
     mailer: str = Field(default="console", description="console | smtp")
     smtp_host: str | None = None