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/application/upload_service.py

@@ -92,7 +92,7 @@ class UploadService:
                     id=track_id,
                     title=title,
                     artist_id=artist.id,
-                    file_path=key,
+                    storage_uri=key,
                     file_format=ext,
                     file_size=file_size,
                     source="upload",