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/streaming_service.py

@@ -73,7 +73,7 @@ class StreamingService:
         if track is None:
             raise NotFoundError("Track not found.")
 
-        stat = await self._storage.stat(track.file_path)
+        stat = await self._storage.stat(track.storage_uri)
         total_size = stat.size
         content_type = stat.content_type or _FORMAT_CONTENT_TYPE.get(
             track.file_format.lower(), "application/octet-stream"
@@ -81,7 +81,7 @@ class StreamingService:
 
         start, end, is_partial = _parse_range(range_header, total_size)
 
-        stream, _ = await self._storage.open_range(track.file_path, start, end)
+        stream, _ = await self._storage.open_range(track.storage_uri, start, end)
 
         actual_end = end if end is not None else total_size - 1
         content_length = actual_end - start + 1