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>

app/domain/entities/track.py

@@ -19,7 +19,7 @@ class Track:
     title: str
     artist_id: uuid.UUID
     album_id: uuid.UUID | None
-    file_path: str
+    storage_uri: str
     file_format: str
     file_size: int
     source: str

app/domain/ports.py

@@ -100,7 +100,7 @@ class TrackRepository(Protocol):
         id: uuid.UUID,
         title: str,
         artist_id: uuid.UUID,
-        file_path: str,
+        storage_uri: str,
         file_format: str,
         file_size: int,
         source: str,