feat(musicbrainz): add smart playlist generation backend (Stage 4)

- Add PlaylistRule enum with BandMembers, ArtistGroups, CollaboratorNetwork variants
- Add GeneratedPlaylist struct for playlist preview results
- Add PlaylistGenerationConfig for customizing track limits
- Implement smart_playlist_preview command to generate playlists from MusicBrainz relationships
- Implement smart_playlist_resolve_artist helper to find artist MBID by name
- Implement smart_playlist_get_available_types to check what playlist types are available
- Search Qobuz for member/collaborator tracks and return matching track IDs

src-tauri/src/commands/mod.rs

@@ -20,6 +20,7 @@ pub mod queue;
 pub mod radio;
 pub mod search;
 pub mod share;
+pub mod smart_playlists;
 
 pub use audio_backends::*;
 pub use audio_diagnostics::*;
@@ -39,3 +40,4 @@ pub use queue::*;
 pub use radio::*;
 pub use search::*;
 pub use share::*;
+pub use smart_playlists::*;