feat(12-01): implement library CRUD methods and orphan cleanup pipeline

- Add AddLibrary, RenameLibrary, RemoveLibrary, GetRemovalImpact methods
- RemoveLibrary follows exact order: phantom populate → delete audio_files → orphan cleanup → commit → FTS5 rebuild → cover art file cleanup → queue compact → events
- Add cancelLibraryScan and currentTrackBelongsToLibrary helpers
- Add RemovalHooks type and removalHooks field on Library struct
- Add LibraryAdded, LibraryRenamed, LibraryRemoved event constants
- Regenerate frontend events.ts with new CRUD events
- All hand-crafted SQL has SAFETY comments per project convention
- Dual artist_credit FK check (recordings + release_groups) for orphan cleanup

backend/library/library.go

@@ -93,6 +93,10 @@ type Library struct {
 	scanQueue              []scanQueueEntry
 	currentScanLibraryID   int64
 	currentScanLibraryName string
+
+	// removalHooks holds callbacks for cross-cutting concerns during
+	// library removal (e.g. stopping playback, compacting queue).
+	removalHooks RemovalHooks
 }
 
 // SetRescanHooks provides optional hooks for cross-cutting