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/gemini_v1beta_handler.go

@@ -212,7 +212,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	if sessionHash != "" {
 		sessionKey = "gemini:" + sessionHash
 	}
-	const maxAccountSwitches = 3
+	maxAccountSwitches := h.maxAccountSwitchesGemini
 	switchCount := 0
 	failedAccountIDs := make(map[int64]struct{})
 	lastFailoverStatus := 0