feat(channel): 渠道管理全链路集成 — 模型映射、定价、限制、用量统计

- 渠道模型映射：支持精确匹配和通配符映射，按平台隔离
- 渠道模型定价：支持 token/按次/图片三种计费模式，区间分层定价
- 模型限制：渠道可限制仅允许定价列表中的模型
- 计费模型来源：支持 requested/upstream 两种计费模型选择
- 用量统计：usage_logs 新增 channel_id/model_mapping_chain/billing_tier/billing_mode 字段
- Dashboard 支持 model_source 维度（requested/upstream/mapping）查看模型统计
- 全部 gateway handler 统一接入 ResolveChannelMappingAndRestrict
- 修复测试：同步 SoraGenerationRepository 接口、SQL INSERT 参数、scan 字段

backend/internal/pkg/usagestats/usage_log_types.go

@@ -175,6 +175,13 @@ type UserBreakdownDimension struct {
 	ModelType    string // "requested", "upstream", or "mapping"
 	Endpoint     string // filter by endpoint value (non-empty to enable)
 	EndpointType string // "inbound", "upstream", or "path"
+	// Additional filter conditions
+	UserID      int64  // filter by user_id (>0 to enable)
+	APIKeyID    int64  // filter by api_key_id (>0 to enable)
+	AccountID   int64  // filter by account_id (>0 to enable)
+	RequestType *int16 // filter by request_type (non-nil to enable)
+	Stream      *bool  // filter by stream flag (non-nil to enable)
+	BillingType *int8  // filter by billing_type (non-nil to enable)
 }
 
 // APIKeyUsageTrendPoint represents API key usage trend data point