feat(dashboard): add model source dimension to stats queries

Support querying model statistics by 'requested', 'upstream', or 'mapping' dimension. Add resolveModelDimensionExpression for safe SQL expression generation, IsValidModelSource whitelist validator, and NormalizeModelSource fallback. Repository persists and scans upstream_model in all insert/select paths.

backend/internal/pkg/usagestats/usage_log_types.go

@@ -3,6 +3,28 @@ package usagestats
 
 import "time"
 
+const (
+	ModelSourceRequested = "requested"
+	ModelSourceUpstream  = "upstream"
+	ModelSourceMapping   = "mapping"
+)
+
+func IsValidModelSource(source string) bool {
+	switch source {
+	case ModelSourceRequested, ModelSourceUpstream, ModelSourceMapping:
+		return true
+	default:
+		return false
+	}
+}
+
+func NormalizeModelSource(source string) string {
+	if IsValidModelSource(source) {
+		return source
+	}
+	return ModelSourceRequested
+}
+
 // DashboardStats 仪表盘统计
 type DashboardStats struct {
 	// 用户统计
@@ -143,6 +165,7 @@ type UserBreakdownItem struct {
 type UserBreakdownDimension struct {
 	GroupID      int64  // filter by group_id (>0 to enable)
 	Model        string // filter by model name (non-empty to enable)
+	ModelType    string // "requested", "upstream", or "mapping"
 	Endpoint     string // filter by endpoint value (non-empty to enable)
 	EndpointType string // "inbound", "upstream", or "path"
 }