feat(gateway): 账户切换次数和 Antigravity 限流时间可配置

- gateway.max_account_switches: 账户切换最大次数，默认 10
- gateway.max_account_switches_gemini: Gemini 账户切换次数，默认 3
- gateway.antigravity_fallback_cooldown_minutes: Antigravity 429 fallback 限流时间，默认 5 分钟
- Antigravity 429 不再重试，直接标记账户限流

backend/internal/handler/openai_gateway_handler.go

@@ -23,6 +23,7 @@ type OpenAIGatewayHandler struct {
 	gatewayService      *service.OpenAIGatewayService
 	billingCacheService *service.BillingCacheService
 	concurrencyHelper   *ConcurrencyHelper
+	maxAccountSwitches  int
 }
 
 // NewOpenAIGatewayHandler creates a new OpenAIGatewayHandler
@@ -33,13 +34,18 @@ func NewOpenAIGatewayHandler(
 	cfg *config.Config,
 ) *OpenAIGatewayHandler {
 	pingInterval := time.Duration(0)
+	maxAccountSwitches := 3
 	if cfg != nil {
 		pingInterval = time.Duration(cfg.Concurrency.PingInterval) * time.Second
+		if cfg.Gateway.MaxAccountSwitches > 0 {
+			maxAccountSwitches = cfg.Gateway.MaxAccountSwitches
+		}
 	}
 	return &OpenAIGatewayHandler{
 		gatewayService:      gatewayService,
 		billingCacheService: billingCacheService,
 		concurrencyHelper:   NewConcurrencyHelper(concurrencyService, SSEPingFormatComment, pingInterval),
+		maxAccountSwitches:  maxAccountSwitches,
 	}
 }
 
@@ -147,7 +153,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 	// Generate session hash (from header for OpenAI)
 	sessionHash := h.gatewayService.GenerateSessionHash(c)
 
-	const maxAccountSwitches = 3
+	maxAccountSwitches := h.maxAccountSwitches
 	switchCount := 0
 	failedAccountIDs := make(map[int64]struct{})
 	lastFailoverStatus := 0