记账: Recorder Close 幂等防重复关闭

- main 的 defer Shutdown 与显式 Close 双调用导致停机 panic close of closed channel
- 用 sync.Once 保证只关闭一次

Co-Authored-By: Claude <noreply@anthropic.com>

server/internal/usage/usage.go

@@ -18,6 +18,7 @@ type Recorder struct {
 	ch     chan *store.UsageLog
 	wg     sync.WaitGroup
 	closed chan struct{}
+	once   sync.Once // 保证 Close 只执行一次（main 的 defer Shutdown 与显式 Close 双调用）
 }
 
 const batchSize = 32
@@ -45,9 +46,11 @@ func (r *Recorder) Record(l *store.UsageLog) {
 }
 
 func (r *Recorder) Close() {
+	r.once.Do(func() {
 		close(r.closed)
 		r.wg.Wait()
 		close(r.ch)
+	})
 }
 
 func (r *Recorder) run() {