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>

alembic/versions/20260608_1132-rename_track_file_path_to_storage_uri.py

@@ -0,0 +1,25 @@
+"""rename track file_path to storage_uri
+
+Revision ID: 20260608_storage_uri
+Revises: e670d6c41d0c
+Create Date: 2026-06-08 11:32:00.000000
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "20260608_storage_uri"
+down_revision: str | None = "e670d6c41d0c"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.alter_column("tracks", "file_path", new_column_name="storage_uri")
+
+
+def downgrade() -> None:
+    op.alter_column("tracks", "storage_uri", new_column_name="file_path")