feat: add linear delay between Antigravity account failover switches

backend/internal/handler/gateway_handler.go

@@ -350,6 +350,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					}
 					switchCount++
 					log.Printf("Account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
+					if account.Platform == service.PlatformAntigravity {
+						if !sleepFailoverDelay(c.Request.Context(), switchCount) {
+							return
+						}
+					}
 					continue
 				}
 				// 错误响应已在Forward中处理，这里只记录日志
@@ -545,6 +550,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					}
 					switchCount++
 					log.Printf("Account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
+					if account.Platform == service.PlatformAntigravity {
+						if !sleepFailoverDelay(c.Request.Context(), switchCount) {
+							return
+						}
+					}
 					continue
 				}
 				// 错误响应已在Forward中处理，这里只记录日志
@@ -807,6 +817,21 @@ func (h *GatewayHandler) handleConcurrencyError(c *gin.Context, err error, slotT
 		fmt.Sprintf("Concurrency limit exceeded for %s, please retry later", slotType), streamStarted)
 }
 
+// sleepFailoverDelay 账号切换线性递增延时：第1次0s、第2次1s、第3次2s…
+// 返回 false 表示 context 已取消。
+func sleepFailoverDelay(ctx context.Context, switchCount int) bool {
+	delay := time.Duration(switchCount-1) * time.Second
+	if delay <= 0 {
+		return true
+	}
+	select {
+	case <-ctx.Done():
+		return false
+	case <-time.After(delay):
+		return true
+	}
+}
+
 func (h *GatewayHandler) handleFailoverExhausted(c *gin.Context, failoverErr *service.UpstreamFailoverError, platform string, streamStarted bool) {
 	statusCode := failoverErr.StatusCode
 	responseBody := failoverErr.ResponseBody

backend/internal/handler/gemini_v1beta_handler.go

@@ -433,6 +433,11 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				lastFailoverErr = failoverErr
 				switchCount++
 				log.Printf("Gemini account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
+				if account.Platform == service.PlatformAntigravity {
+					if !sleepFailoverDelay(c.Request.Context(), switchCount) {
+						return
+					}
+				}
 				continue
 			}
 			// ForwardNative already wrote the response