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

build.rs

@@ -0,0 +1,7 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tonic_build::configure()
+        .build_server(false)
+        .build_client(true)
+        .compile_protos(&["proto/metadata/v1/metadata.proto"], &["proto"])?;
+    Ok(())
+}