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/pyproject.toml

@@ -12,6 +12,8 @@ dependencies = [
     "asyncpg>=0.30",
     "alembic>=1.14",
     "argon2-cffi>=23.1",
+    "boto3>=1.35",
+    "python-multipart>=0.0.12",
 ]
 
 [dependency-groups]
@@ -36,6 +38,10 @@ extend-exclude = ["migrations/versions"]
 [tool.ruff.lint]
 select = ["E", "F", "I", "UP", "B"]
 
+[tool.ruff.lint.flake8-bugbear]
+# FastAPI uses these as call-expressions in argument defaults by design.
+extend-immutable-calls = ["fastapi.File", "fastapi.Form", "fastapi.Depends", "fastapi.Query", "fastapi.Header"]
+
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 pythonpath = ["."]