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/admin/group_handler.go

@@ -302,3 +302,36 @@ func (h *GroupHandler) GetGroupAPIKeys(c *gin.Context) {
 	}
 	response.Paginated(c, outKeys, total, page, pageSize)
 }
+
+// UpdateSortOrderRequest represents the request to update group sort orders
+type UpdateSortOrderRequest struct {
+	Updates []struct {
+		ID        int64 `json:"id" binding:"required"`
+		SortOrder int   `json:"sort_order"`
+	} `json:"updates" binding:"required,min=1"`
+}
+
+// UpdateSortOrder handles updating group sort orders
+// PUT /api/v1/admin/groups/sort-order
+func (h *GroupHandler) UpdateSortOrder(c *gin.Context) {
+	var req UpdateSortOrderRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "Invalid request: "+err.Error())
+		return
+	}
+
+	updates := make([]service.GroupSortOrderUpdate, 0, len(req.Updates))
+	for _, u := range req.Updates {
+		updates = append(updates, service.GroupSortOrderUpdate{
+			ID:        u.ID,
+			SortOrder: u.SortOrder,
+		})
+	}
+
+	if err := h.adminService.UpdateGroupSortOrders(c.Request.Context(), updates); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, gin.H{"message": "Sort order updated successfully"})
+}