chore(dev): run backend with air

AGENTS.md

@@ -22,7 +22,7 @@ The frontend builds to `frontend/public/dist/`, then `make frontend` copies the
 make all        # Build frontend + backend
 make frontend   # Build frontend, copy to backend/web/static/
 make backend    # Build Go binary (requires frontend assets)
-make dev        # Run both with hot-reload
+make dev        # Run frontend watcher + backend hot reload via air
 make tests      # Run Go tests
 make docker     # Build Docker image
 ```