Merge PR #142: feat: 多平台网关优化与用户系统增强

主要功能：
- Gemini OAuth 配额系统优化：Google One tier 自动推断
- Antigravity 网关增强：Thinking Block 重试、Claude 模型 signature 透传
- 账号调度改进：临时不可调度功能、负载感知调度优化
- 前端用户体验：账号管理界面优化、使用教程改进

冲突解决：保留 handleUpstreamError 的 prefix 参数和日志记录能力，
同时合并 PR 的 thinking block 重试和 model fallback 功能。

backend/internal/handler/openai_gateway_handler.go

@@ -41,7 +41,7 @@ func NewOpenAIGatewayHandler(
 // POST /openai/v1/responses
 func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 	// Get apiKey and user from context (set by ApiKeyAuth middleware)
-	apiKey, ok := middleware2.GetApiKeyFromContext(c)
+	apiKey, ok := middleware2.GetAPIKeyFromContext(c)
 	if !ok {
 		h.errorResponse(c, http.StatusUnauthorized, "authentication_error", "Invalid API key")
 		return
@@ -235,7 +235,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 			defer cancel()
 			if err := h.gatewayService.RecordUsage(ctx, &service.OpenAIRecordUsageInput{
 				Result:       result,
-				ApiKey:       apiKey,
+				APIKey:       apiKey,
 				User:         apiKey.User,
 				Account:      usedAccount,
 				Subscription: subscription,