fix: flaky WebSocket test, usage request queue, and test improvements

- Fix flaky WebSocket passthrough test: allow StatusNormalClosure after
  client close instead of requiring NoError (race condition fix)
- Fix ratelimit 401 test: use PlatformOpenAI instead of PlatformGemini
  for OAuth token cache invalidation scenario (more accurate)
- Add usageLoadQueue: Anthropic OAuth/setup-token accounts sharing the
  same proxy exit are serialized with 1-2s jitter to prevent upstream 429
- AccountUsageCell: add module-level usage cache (5min TTL), unmounted
  safety guard, and integrate enqueueUsageRequest for throttled fetching

backend/internal/service/openai_ws_forwarder_ingress_session_test.go

@@ -413,7 +413,12 @@ func TestOpenAIGatewayService_ProxyResponsesWebSocketFromClient_PassthroughModeR
 
 	select {
 	case serverErr := <-serverErrCh:
-		require.NoError(t, serverErr)
+		// After normal client close, the server goroutine may receive the close frame
+		// as an error — this is expected behavior, not a test failure.
+		if serverErr != nil {
+			require.Contains(t, serverErr.Error(), "StatusNormalClosure",
+				"server error should only be a normal close frame, got: %v", serverErr)
+		}
 	case <-time.After(5 * time.Second):
 		t.Fatal("等待 passthrough websocket 结束超时")
 	}

backend/internal/service/ratelimit_service_401_test.go

@@ -102,6 +102,8 @@ func TestRateLimitService_HandleUpstreamError_OAuth401SetsTempUnschedulable(t *t
 	})
 }
 
+// TestRateLimitService_HandleUpstreamError_OAuth401InvalidatorError
+// OpenAI OAuth 401 缓存失效出错时仍走 temp_unschedulable
 func TestRateLimitService_HandleUpstreamError_OAuth401InvalidatorError(t *testing.T) {
 	repo := &rateLimitAccountRepoStub{}
 	invalidator := &tokenCacheInvalidatorRecorder{err: errors.New("boom")}
@@ -109,7 +111,7 @@ func TestRateLimitService_HandleUpstreamError_OAuth401InvalidatorError(t *testin
 	service.SetTokenCacheInvalidator(invalidator)
 	account := &Account{
 		ID:       101,
-		Platform: PlatformGemini,
+		Platform: PlatformOpenAI,
 		Type:     AccountTypeOAuth,
 	}