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

config.example.yaml

@@ -1,6 +1,9 @@
 database:
   url: "postgresql://music:music@localhost:5433/music_aggregator"
 
+metadata:
+  endpoint: "http://localhost:50051"
+
 indexers:
   - name: "Jackett"
     url: "http://localhost:9117"