Add fix actions for maintenance findings (dead files, orphans, duplicates, etc.)
- New fix_finding() dispatcher in repair_worker with per-type handlers:
dead_file (remove DB entry), orphan_file (delete from disk),
track_number_mismatch (update DB), missing_cover_art (apply artwork URL),
metadata_gap (apply found fields), duplicate_tracks (keep best quality)
- New POST /api/repair/findings/<id>/fix endpoint
- Frontend: contextual fix buttons per finding type, bulk "Fix Selected"
- Removed path_mismatch from fixable types (dry-run preview only)
7a706e8c11