feat(api): expose model_source filter in dashboard endpoints

Add model_source query parameter to GetModelStats and GetUserBreakdown handlers with explicit IsValidModelSource validation. Include model_source in cache key to prevent cross-source cache hits. Expose upstream_model in usage log DTO with omitempty semantics.

backend/internal/handler/admin/dashboard_snapshot_v2_handler.go

@@ -200,6 +200,7 @@ func (h *DashboardHandler) buildSnapshotV2Response(
 			filters.APIKeyID,
 			filters.AccountID,
 			filters.GroupID,
+			usagestats.ModelSourceRequested,
 			filters.RequestType,
 			filters.Stream,
 			filters.BillingType,