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/internal/handler/dto/mappers.go

@@ -115,6 +115,7 @@ func GroupFromServiceAdmin(g *service.Group) *AdminGroup {
 		MCPXMLInject:         g.MCPXMLInject,
 		SupportedModelScopes: g.SupportedModelScopes,
 		AccountCount:         g.AccountCount,
+		SortOrder:            g.SortOrder,
 	}
 	if len(g.AccountGroups) > 0 {
 		out.AccountGroups = make([]AccountGroup, 0, len(g.AccountGroups))

backend/internal/handler/dto/types.go

@@ -98,6 +98,9 @@ type AdminGroup struct {
 	SupportedModelScopes []string       `json:"supported_model_scopes"`
 	AccountGroups        []AccountGroup `json:"account_groups,omitempty"`
 	AccountCount         int64          `json:"account_count,omitempty"`
+
+	// 分组排序
+	SortOrder int `json:"sort_order"`
 }
 
 type Account struct {