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/group/group.go

@@ -63,6 +63,8 @@ const (
 	FieldMcpXMLInject = "mcp_xml_inject"
 	// FieldSupportedModelScopes holds the string denoting the supported_model_scopes field in the database.
 	FieldSupportedModelScopes = "supported_model_scopes"
+	// FieldSortOrder holds the string denoting the sort_order field in the database.
+	FieldSortOrder = "sort_order"
 	// EdgeAPIKeys holds the string denoting the api_keys edge name in mutations.
 	EdgeAPIKeys = "api_keys"
 	// EdgeRedeemCodes holds the string denoting the redeem_codes edge name in mutations.
@@ -162,6 +164,7 @@ var Columns = []string{
 	FieldModelRoutingEnabled,
 	FieldMcpXMLInject,
 	FieldSupportedModelScopes,
+	FieldSortOrder,
 }
 
 var (
@@ -225,6 +228,8 @@ var (
 	DefaultMcpXMLInject bool
 	// DefaultSupportedModelScopes holds the default value on creation for the "supported_model_scopes" field.
 	DefaultSupportedModelScopes []string
+	// DefaultSortOrder holds the default value on creation for the "sort_order" field.
+	DefaultSortOrder int
 )
 
 // OrderOption defines the ordering options for the Group queries.
@@ -345,6 +350,11 @@ func ByMcpXMLInject(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldMcpXMLInject, opts...).ToFunc()
 }
 
+// BySortOrder orders the results by the sort_order field.
+func BySortOrder(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldSortOrder, opts...).ToFunc()
+}
+
 // ByAPIKeysCount orders the results by api_keys count.
 func ByAPIKeysCount(opts ...sql.OrderTermOption) OrderOption {
 	return func(s *sql.Selector) {