feat: 实现文件夹和书签的持久排序与拖拽功能

openspec/changes/add-dnd-sorting/specs/api/spec.md

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+## ADDED Requirements
+
+### Requirement: Folder ordering persistence
+The system SHALL persist folder ordering per user per parent folder.
+
+#### Scenario: List folders returns stable ordered result
+- **GIVEN** an authenticated user
+- **WHEN** the user calls `GET /folders`
+- **THEN** the server returns folders ordered by `(parentId, sortOrder, name)`
+
+#### Scenario: Reorder folders within the same parent
+- **GIVEN** an authenticated user
+- **WHEN** the user calls `POST /folders/reorder` with `parentId` and `orderedIds`
+- **THEN** the server persists the new order and returns `{ ok: true }`
+
+### Requirement: Bookmark ordering persistence
+The system SHALL persist bookmark ordering per user per folder.
+
+#### Scenario: List my bookmarks returns stable ordered result
+- **GIVEN** an authenticated user
+- **WHEN** the user calls `GET /bookmarks`
+- **THEN** the server returns bookmarks ordered by `(folderId, sortOrder, updatedAt desc)`
+
+#### Scenario: Reorder bookmarks within the same folder
+- **GIVEN** an authenticated user
+- **WHEN** the user calls `POST /bookmarks/reorder` with `folderId` and `orderedIds`
+- **THEN** the server persists the new order and returns `{ ok: true }`
+
+### Requirement: Root group treated consistently
+The system SHALL treat `folderId=null` bookmarks as belonging to the root group.
+
+#### Scenario: Reorder root-group bookmarks
+- **GIVEN** an authenticated user
+- **WHEN** the user calls `POST /bookmarks/reorder` with `folderId=null`
+- **THEN** the server reorders root-group bookmarks and returns `{ ok: true }`