feat: add metadata-agregator gRPC client integration

- Add tonic/prost for gRPC client generation
- Add proto definitions from metadata-agregator service
- Add MetadataClient and MetadataService for gRPC communication
- Add REST controller exposing metadata endpoints (search, get, sync)
- Update config with metadata.endpoint setting
- Update flake.nix with protobuf for proto compilation

src/api/mod.rs

@@ -1,4 +1,5 @@
 mod indexer_controller;
+mod metadata_controller;
 mod torrent_controller;
 
 use axum::{
@@ -23,6 +24,7 @@ pub fn routes(state: AppState) -> Router {
         .route("/stats", get(get_stats))
         .nest("/indexers", indexer_controller::routes())
         .nest("/torrents", torrent_controller::routes())
+        .nest("/metadata", metadata_controller::routes())
         .with_state(state)
 }