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/media.py

@@ -0,0 +1,36 @@
+"""Media — a binary asset (image, scan, PDF, audio) in object storage. The row
+holds metadata + checksum + the storage key; the bytes live in the ObjectStore.
+Optionally attached to a single fact (person, event, or source) for now."""
+
+import uuid
+
+from sqlalchemy import BigInteger, ForeignKey, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base
+from app.models.mixins import SoftDelete, TenantScoped, Timestamps, UUIDPrimaryKey
+
+
+class Media(Base, UUIDPrimaryKey, TenantScoped, Timestamps, SoftDelete):
+    __tablename__ = "media"
+
+    uploader_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("users.id", ondelete="SET NULL"), index=True
+    )
+    storage_key: Mapped[str] = mapped_column(String(512), unique=True)
+    original_filename: Mapped[str] = mapped_column(String(512))
+    content_type: Mapped[str] = mapped_column(String(128))
+    byte_size: Mapped[int] = mapped_column(BigInteger)
+    checksum_sha256: Mapped[str] = mapped_column(String(64), index=True)
+    title: Mapped[str | None] = mapped_column(String(512))
+
+    # Optional single attachment target.
+    person_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("persons.id", ondelete="SET NULL"), index=True
+    )
+    event_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("events.id", ondelete="SET NULL"), index=True
+    )
+    source_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("sources.id", ondelete="SET NULL"), index=True
+    )