chore: add hot reload for development

- Makefile: add dev-backend (air) and dev-frontend (vite) targets
- backend/.air.toml: air config for Go live reload
- .gitignore: exclude backend/tmp/

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build run test clean fmt lint frontend
+.PHONY: build run test clean fmt lint frontend dev dev-backend dev-frontend
 
 BINARY_NAME=openteam
 BUILD_DIR=bin
@@ -18,9 +18,17 @@ frontend:
 run: build
 	./$(BUILD_DIR)/$(BINARY_NAME)
 
-# Development run
-dev:
-	cd $(BACKEND_DIR) && go run ./cmd/openteam
+# Development: backend + frontend (requires air + pnpm)
+dev: dev-backend dev-frontend
+
+# Go backend with hot reload (requires: go install github.com/air-verse/air@latest)
+dev-backend:
+	@command -v air >/dev/null 2>&1 || { echo "Installing air..."; go install github.com/air-verse/air@latest; }
+	cd $(BACKEND_DIR) && air -c .air.toml
+
+# Vue frontend with HMR
+dev-frontend:
+	cd frontend && pnpm dev
 
 # Test
 test: