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/schema/group.go

@@ -121,6 +121,11 @@ func (Group) Fields() []ent.Field {
 			Default([]string{"claude", "gemini_text", "gemini_image"}).
 			SchemaType(map[string]string{dialect.Postgres: "jsonb"}).
 			Comment("支持的模型系列：claude, gemini_text, gemini_image"),
+
+		// 分组排序 (added by migration 052)
+		field.Int("sort_order").
+			Default(0).
+			Comment("分组显示排序，数值越小越靠前"),
 	}
 }
 
@@ -149,5 +154,6 @@ func (Group) Indexes() []ent.Index {
 		index.Fields("subscription_type"),
 		index.Fields("is_exclusive"),
 		index.Fields("deleted_at"),
+		index.Fields("sort_order"),
 	}
 }