feat(storage): S3-compatible storage adapter + storage_uri rename

Add S3FileStorage adapter (any S3-compatible backend: AWS, MinIO, Garage)
alongside the local adapter, selected via STORAGE_BACKEND. Proxied range
streaming via get_object+Range; as_local_path downloads to a tempfile for
ffmpeg/fpcalc. Rename track.file_path -> storage_uri across domain entity,
ORM, repositories, port, and services, with an Alembic migration. Adds
mocked S3 unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -23,6 +23,8 @@ dependencies = [
     "pwdlib[argon2]>=0.2.1",
     # outbound http (ML client, MusicBrainz, AcoustID)
     "httpx>=0.28",
+    # S3-compatible object storage
+    "aioboto3>=13.0",
     # logging
     "structlog>=24.4",
 ]
@@ -80,6 +82,10 @@ disallow_untyped_defs = true
 module = ["arq.*"]
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = ["aioboto3.*", "aiobotocore.*", "botocore.*"]
+ignore_missing_imports = true
+
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]