fix: add fallback pricing for opus-4.6 and gemini-3.1-pro models

backend/internal/service/billing_service.go

@@ -133,6 +133,18 @@ func (s *BillingService) initFallbackPricing() {
 		CacheReadPricePerToken:     0.03e-6, // $0.03 per MTok
 		SupportsCacheBreakdown:     false,
 	}
+
+	// Claude 4.6 Opus (与4.5同价)
+	s.fallbackPrices["claude-opus-4.6"] = s.fallbackPrices["claude-opus-4.5"]
+
+	// Gemini 3.1 Pro
+	s.fallbackPrices["gemini-3.1-pro"] = &ModelPricing{
+		InputPricePerToken:         2e-6,  // $2 per MTok
+		OutputPricePerToken:        12e-6, // $12 per MTok
+		CacheCreationPricePerToken: 2e-6,  // $2 per MTok
+		CacheReadPricePerToken:     0.2e-6, // $0.20 per MTok
+		SupportsCacheBreakdown:     false,
+	}
 }
 
 // getFallbackPricing 根据模型系列获取回退价格
@@ -141,6 +153,9 @@ func (s *BillingService) getFallbackPricing(model string) *ModelPricing {
 
 	// 按模型系列匹配
 	if strings.Contains(modelLower, "opus") {
+		if strings.Contains(modelLower, "4.6") || strings.Contains(modelLower, "4-6") {
+			return s.fallbackPrices["claude-opus-4.6"]
+		}
 		if strings.Contains(modelLower, "4.5") || strings.Contains(modelLower, "4-5") {
 			return s.fallbackPrices["claude-opus-4.5"]
 		}
@@ -158,6 +173,9 @@ func (s *BillingService) getFallbackPricing(model string) *ModelPricing {
 		}
 		return s.fallbackPrices["claude-3-haiku"]
 	}
+	if strings.Contains(modelLower, "gemini-3.1-pro") || strings.Contains(modelLower, "gemini-3-1-pro") {
+		return s.fallbackPrices["gemini-3.1-pro"]
+	}
 
 	// 默认使用Sonnet价格
 	return s.fallbackPrices["claude-sonnet-4"]