feat(billing): 添加 Gemini 200K 长上下文双倍计费功能

- 新增 CalculateCostWithLongContext 方法支持阈值双倍计费
- 新增 RecordUsageWithLongContext 方法专用于 Gemini 计费
- Gemini 超过 200K token 的部分按 2 倍费率计算
- 其他平台（Claude/OpenAI）完全不受影响

backend/internal/handler/gemini_v1beta_handler.go

@@ -366,18 +366,21 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		userAgent := c.GetHeader("User-Agent")
 		clientIP := ip.GetClientIP(c)
 
-		// 6) record usage async
+		// 6) record usage async (Gemini 使用长上下文双倍计费)
 		go func(result *service.ForwardResult, usedAccount *service.Account, ua, ip string) {
 			ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 			defer cancel()
-			if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
-				Result:       result,
-				APIKey:       apiKey,
-				User:         apiKey.User,
-				Account:      usedAccount,
-				Subscription: subscription,
-				UserAgent:    ua,
-				IPAddress:    ip,
+
+			if err := h.gatewayService.RecordUsageWithLongContext(ctx, &service.RecordUsageLongContextInput{
+				Result:                result,
+				APIKey:                apiKey,
+				User:                  apiKey.User,
+				Account:               usedAccount,
+				Subscription:          subscription,
+				UserAgent:             ua,
+				IPAddress:             ip,
+				LongContextThreshold:  200000, // Gemini 200K 阈值
+				LongContextMultiplier: 2.0,    // 超出部分双倍计费
 			}); err != nil {
 				log.Printf("Record usage failed: %v", err)
 			}