fix(mpc-system): fix protobuf generation in Makefile to generate gRPC service files

- Add mkdir commands to create output directories
- Add paths=source_relative options for go_out and go-grpc_out
- This ensures *_grpc.pb.go files are generated correctly
- Fixes session-coordinator and message-router startup failures

Related: MPC services were failing to start due to missing gRPC service interface files

backend/.claude/settings.local.json

@@ -0,0 +1,11 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git add:*)",
+      "Bash(git commit:*)",
+      "Bash(git push:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}

backend/mpc-system/Makefile

@@ -37,7 +37,11 @@ init: ## Initialize the project (install tools)
 
 proto: ## Generate protobuf code
 	@echo "Generating protobuf..."
-	$(PROTOC) --go_out=. --go-grpc_out=. api/proto/*.proto
+	@mkdir -p api/grpc/coordinator/v1
+	@mkdir -p api/grpc/router/v1
+	$(PROTOC) --go_out=. --go_opt=paths=source_relative \
+		--go-grpc_out=. --go-grpc_opt=paths=source_relative \
+		api/proto/*.proto
 	@echo "Protobuf generated successfully!"
 
 fmt: ## Format Go code