feat(grpc): implement MetadataService handlers

- Implement all 5 RPCs (Get, Update, Clear, Batch, Import)
- Add MetadataServiceImpl with database integration
- Add 10 comprehensive unit tests
- All 19 tests pass, full workspace compiles

crates/musicfs-grpc/src/lib.rs

@@ -6,10 +6,13 @@ pub mod proto {
     }
 }
 
+mod metadata;
 mod search_service;
 mod server;
 mod webhook;
 
+pub use metadata::MetadataServiceImpl;
+pub use proto::musicfs::v1::metadata_service_server::MetadataServiceServer;
 pub use proto::musicfs::v1::music_fs_server::{MusicFs, MusicFsServer as MusicFsGrpcServer};
 pub use proto::musicfs::v1::*;
 pub use search_service::SearchService;