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>
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