feat(grok): 加固媒体计费门控并补齐分组按模型价输入

生成类媒体仅在存在真实 image/video 计费单元时写 usage，避免空结果误扣费。
Create/Update 分组输入支持 video_model_prices，与仓储 JSONB 字段打通。
本阶段不引入假 voice 路由（上游音频路径依赖更广协议层）。

backend/internal/handler/grok_media.go

@@ -417,7 +417,7 @@ func (h *OpenAIGatewayHandler) handleGrokMedia(c *gin.Context, endpoint service.
 				)
 			}
 		}
-		if shouldRecordGrokMediaUsage(endpoint, requestModel) {
+		if shouldRecordGrokMediaUsage(endpoint, requestModel, result) {
 			recordGrokMediaUsage(c, h, reqLog, apiKey, subject, subscription, account, result, requestModel, body, requestID)
 		}
 		reqLog.Debug("grok_media.request_completed",
@@ -459,8 +459,20 @@ func grokMediaScheduleModel(account *service.Account, routingModel string, resul
 	return account.GetMappedModel(routingModel)
 }
 
-func shouldRecordGrokMediaUsage(endpoint service.GrokMediaEndpoint, requestModel string) bool {
-	return endpoint.IsGenerationRequest() && strings.TrimSpace(requestModel) != ""
+// shouldRecordGrokMediaUsage gates usage writes for Grok media generation only.
+// Status/content polls, empty model, and failed generations with zero billable
+// image/video units never bill.
+func shouldRecordGrokMediaUsage(endpoint service.GrokMediaEndpoint, requestModel string, result *service.OpenAIForwardResult) bool {
+	if !endpoint.IsGenerationRequest() || strings.TrimSpace(requestModel) == "" {
+		return false
+	}
+	if result == nil {
+		return false
+	}
+	if result.VideoCount > 0 {
+		return true
+	}
+	return result.ImageCount > 0
 }
 
 func recordGrokMediaUsage(

backend/internal/handler/grok_media_test.go

@@ -3,6 +3,7 @@ package handler
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
@@ -68,7 +69,18 @@ func TestShouldRecordGrokMediaUsage(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			require.Equal(t, tt.want, shouldRecordGrokMediaUsage(tt.endpoint, tt.model))
+			// Nil result must never bill.
+			require.False(t, shouldRecordGrokMediaUsage(tt.endpoint, tt.model, nil))
+			// Non-nil result with units only bills generation endpoints with model.
+			result := &service.OpenAIForwardResult{ImageCount: 1, VideoCount: 0}
+			if tt.endpoint.IsGenerationRequest() && strings.TrimSpace(tt.model) != "" {
+				require.Equal(t, tt.want, shouldRecordGrokMediaUsage(tt.endpoint, tt.model, result))
+			} else {
+				require.False(t, shouldRecordGrokMediaUsage(tt.endpoint, tt.model, result))
+			}
+			// Zero billable units never bill even for generation + model.
+			empty := &service.OpenAIForwardResult{}
+			require.False(t, shouldRecordGrokMediaUsage(tt.endpoint, tt.model, empty))
 		})
 	}
 }

backend/internal/service/admin_group.go

@@ -476,6 +476,7 @@ func (s *adminServiceImpl) CreateGroup(ctx context.Context, input *CreateGroupIn
 		VideoPrice480P:                  videoPrice480P,
 		VideoPrice720P:                  videoPrice720P,
 		VideoPrice1080P:                 videoPrice1080P,
+		VideoModelPrices:                NormalizeVideoModelPrices(input.VideoModelPrices),
 		WebSearchPricePerCall:           webSearchPricePerCall,
 		ClaudeCodeOnly:                  input.ClaudeCodeOnly,
 		FallbackGroupID:                 input.FallbackGroupID,
@@ -755,6 +756,10 @@ func (s *adminServiceImpl) UpdateGroup(ctx context.Context, id int64, input *Upd
 	if input.VideoPrice1080P != nil {
 		group.VideoPrice1080P = normalizePrice(input.VideoPrice1080P)
 	}
+	// nil = leave unchanged; empty map = clear per-model prices.
+	if input.VideoModelPrices != nil {
+		group.VideoModelPrices = NormalizeVideoModelPrices(input.VideoModelPrices)
+	}
 	if input.WebSearchPricePerCall != nil {
 		group.WebSearchPricePerCall = normalizePrice(input.WebSearchPricePerCall)
 	}

backend/internal/service/admin_service.go

@@ -238,6 +238,8 @@ type CreateGroupInput struct {
 	VideoPrice480P     *float64
 	VideoPrice720P     *float64
 	VideoPrice1080P    *float64
+	// VideoModelPrices 可选按模型族×分辨率覆盖视频每秒单价。
+	VideoModelPrices map[string]map[string]float64
 	// Codex alpha/search 网页搜索单次价格（USD/次，仅 openai 平台使用）；nil/负数按默认价 0.01 处理
 	WebSearchPricePerCall *float64
 	ClaudeCodeOnly        bool   // 仅允许 Claude Code 客户端
@@ -303,6 +305,8 @@ type UpdateGroupInput struct {
 	VideoPrice480P     *float64
 	VideoPrice720P     *float64
 	VideoPrice1080P    *float64
+	// VideoModelPrices 可选按模型族×分辨率覆盖；nil 表示不修改，空 map 表示清除。
+	VideoModelPrices map[string]map[string]float64
 	// Codex alpha/search 网页搜索单次价格（USD/次）；nil 表示不修改，负数表示清除回默认价 0.01
 	WebSearchPricePerCall *float64
 	ClaudeCodeOnly        *bool  // 仅允许 Claude Code 客户端