feat(storage): wire Storage Maintenance screen (A6)

Duplicates / failed / missing-metadata tabs (paginated, reusing TrackRow) plus
admin orphan-file cleanup. Adds getDuplicates/getBrokenFiles/getMissingMetadata/
runCleanup endpoints and a DuplicateGroup type; drops the dead scanStorage/
deleteTrackFile stubs that had no backend route.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/types.ts

@@ -176,6 +176,12 @@ export interface StorageStats {
   disk?: StorageDiskUsage;
 }
 
+/** A set of tracks sharing one acoustic fingerprint — de-dup candidates (§A6). */
+export interface DuplicateGroup {
+  fingerprint: string;
+  tracks: Track[];
+}
+
 export interface User {
   id: string;
   username: string;