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/events/events.go

@@ -60,3 +60,10 @@ const (
 	LibraryScanQueued       = "LibraryScanQueued"
 	LibraryScanQueueDrained = "LibraryScanQueueDrained"
 )
+
+// Library CRUD events.
+const (
+	LibraryAdded   = "LibraryAdded"
+	LibraryRenamed = "LibraryRenamed"
+	LibraryRemoved = "LibraryRemoved"
+)