Implement `GET /api/v1/storage`, replacing the stub. Returns aggregate
library facts (track/artist/album counts, total footprint, playtime,
per-format / per-source / metadata-status breakdowns, top genres) plus
the real capacity of the backing volume.
- domain: `LibraryStats`, `FormatBreakdown`, `DiskUsage` value objects
- ports: `FileStorage.disk_usage()` (local = shutil.disk_usage walking up
to the nearest existing ancestor; S3 returns None — no fixed disk)
- repo: `TrackRepository.library_stats()` (single set of GROUP BYs)
- tests: storage stats API (auth, empty library, upload counting)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Senko-san