feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

backend/ent/migrate/schema.go

@@ -372,6 +372,7 @@ var (
 		{Name: "model_routing_enabled", Type: field.TypeBool, Default: false},
 		{Name: "mcp_xml_inject", Type: field.TypeBool, Default: true},
 		{Name: "supported_model_scopes", Type: field.TypeJSON, SchemaType: map[string]string{"postgres": "jsonb"}},
+		{Name: "sort_order", Type: field.TypeInt, Default: 0},
 	}
 	// GroupsTable holds the schema information for the "groups" table.
 	GroupsTable = &schema.Table{
@@ -404,6 +405,11 @@ var (
 				Unique:  false,
 				Columns: []*schema.Column{GroupsColumns[3]},
 			},
+			{
+				Name:    "group_sort_order",
+				Unique:  false,
+				Columns: []*schema.Column{GroupsColumns[25]},
+			},
 		},
 	}
 	// PromoCodesColumns holds the columns for the "promo_codes" table.