protobuf migration: replace JSON with prost-generated wire format

- Expanded protocol/lnex.proto with full message set for all packet types
- Added prost + prost-build to gateway build pipeline
- Refactored all handlers and framing to use prost encode/decode
- Removed serde payload structs (now generated from .proto)
- Server starts and admin endpoint responds

gateway/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    prost_build::compile_protos(&["../protocol/lnex.proto"], &["../protocol/"]).unwrap();
+}