fix(lint): 修复 golangci-lint 报错

- 修复 gofmt 格式问题
- 修复 staticcheck SA4031 nil check 问题（只在成功时设置 release 函数）
- 删除未使用的 sortAccountsByPriority 函数

backend/internal/handler/gateway_handler.go

@@ -192,10 +192,12 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					log.Printf("Account wait queue full: account=%d", account.ID)
 					h.handleStreamingAwareError(c, http.StatusTooManyRequests, "rate_limit_error", "Too many pending requests, please retry later", streamStarted)
 					return
-				}
+				} else {
+					// Only set release function if increment succeeded
 					accountWaitRelease = func() {
 						h.concurrencyHelper.DecrementAccountWaitCount(c.Request.Context(), account.ID)
 					}
+				}
 
 				accountReleaseFunc, err = h.concurrencyHelper.AcquireAccountSlotWithWaitTimeout(
 					c,
@@ -314,10 +316,12 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				log.Printf("Account wait queue full: account=%d", account.ID)
 				h.handleStreamingAwareError(c, http.StatusTooManyRequests, "rate_limit_error", "Too many pending requests, please retry later", streamStarted)
 				return
-			}
+			} else {
+				// Only set release function if increment succeeded
 				accountWaitRelease = func() {
 					h.concurrencyHelper.DecrementAccountWaitCount(c.Request.Context(), account.ID)
 				}
+			}
 
 			accountReleaseFunc, err = h.concurrencyHelper.AcquireAccountSlotWithWaitTimeout(
 				c,

backend/internal/handler/gemini_v1beta_handler.go

@@ -233,10 +233,12 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				log.Printf("Account wait queue full: account=%d", account.ID)
 				googleError(c, http.StatusTooManyRequests, "Too many pending requests, please retry later")
 				return
-			}
+			} else {
+				// Only set release function if increment succeeded
 				accountWaitRelease = func() {
 					geminiConcurrency.DecrementAccountWaitCount(c.Request.Context(), account.ID)
 				}
+			}
 
 			accountReleaseFunc, err = geminiConcurrency.AcquireAccountSlotWithWaitTimeout(
 				c,

backend/internal/service/gateway_service.go

@@ -681,12 +681,6 @@ func (s *GatewayService) tryAcquireAccountSlot(ctx context.Context, accountID in
 	return s.concurrencyService.AcquireAccountSlot(ctx, accountID, maxConcurrency)
 }
 
-func sortAccountsByPriority(accounts []*Account) {
-	sort.SliceStable(accounts, func(i, j int) bool {
-		return accounts[i].Priority < accounts[j].Priority
-	})
-}
-
 func sortAccountsByPriorityAndLastUsed(accounts []*Account, preferOAuth bool) {
 	sort.SliceStable(accounts, func(i, j int) bool {
 		a, b := accounts[i], accounts[j]