feat: P1-3 add heartbeat keep-alive goroutine

- add heartbeat.go with heartbeatIntervalSecs var, SetHeartbeatInterval, runHeartbeat (no build tag, CGO-free)
- add heartbeat_test.go with 5 tests covering interval validation and goroutine lifecycle
- add StartHeartbeat to clib.go (CGO impl calling xcecli_heartbeat)
- remove SetHeartbeatInterval stub from stub.go (now real code in heartbeat.go)
- remove TestSetHeartbeatInterval_StubNoOp from stub_funcs_test.go
- call SetHeartbeatInterval in OnClusterUpdated to track backend-pushed interval
- call backend.StartHeartbeat in cmd/main.go after backend registration

src/frontend/apiserver/cmd/main.go

@@ -9,6 +9,7 @@ import (
 	"apiserver/store"
 	"apiserver/utils"
 	logger "apiserver/utils"
+	"context"
 	"flag"
 	"fmt"
 	"io"
@@ -71,6 +72,7 @@ func main() {
 			os.Exit(1)
 		}
 		logger.Info("********** Backend connection registered!")
+		backend.StartHeartbeat(context.Background())
 	} else {
 		logger.Info("********** Backend not configured — running in stub mode")
 	}