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/migrations/052_add_group_sort_order.sql

@@ -0,0 +1,8 @@
+-- Add sort_order field to groups table for custom ordering
+ALTER TABLE groups ADD COLUMN IF NOT EXISTS sort_order INT NOT NULL DEFAULT 0;
+
+-- Initialize existing groups with sort_order based on their ID
+UPDATE groups SET sort_order = id WHERE sort_order = 0;
+
+-- Create index for efficient sorting
+CREATE INDEX IF NOT EXISTS idx_groups_sort_order ON groups(sort_order);