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/dto/mappers.go

@@ -521,6 +521,7 @@ func usageLogFromServiceUser(l *service.UsageLog) UsageLog {
 		AccountID:             l.AccountID,
 		RequestID:             l.RequestID,
 		Model:                 l.Model,
+		UpstreamModel:         l.UpstreamModel,
 		ServiceTier:           l.ServiceTier,
 		ReasoningEffort:       l.ReasoningEffort,
 		InboundEndpoint:       l.InboundEndpoint,