feat: add WebDAV sync support and startup/shutdown scripts

Backend:
- Add uuid, sync_version, is_deleted fields to all syncable models
- Add SyncSettings model for WebDAV configuration (AES-256-GCM encrypted passwords)
- Add crypto.py: AES-256-GCM encryption derived from JWT_SECRET via PBKDF2
- Add sync_lock.py: thread-level sync lock with 503 middleware for write blocking
- Add webdav.py: WebDAV client using requests (PUT/GET/MKCOL/DELETE)
- Add sync_service.py: push/pull/bidirectional merge with LWW conflict resolution
- Add sync router with 8 endpoints: config, test, push, pull, sync, status, remote delete
- Add UUID backfill for existing records in init_db()
- Add SQLAlchemy before_update event to auto-increment sync_version
- Register sync middleware to block writes during sync (503)

Frontend:
- Add sync API client (WebUI/src/api/sync.ts)
- Add useSyncStore with config, test, push/pull/sync operations
- Add WebDAV config + sync UI in SettingsView
- Add 503 status code handling in axios interceptor
- Add uuid field to all TypeScript type definitions

Scripts:
- Add scripts/start.bat and scripts/stop.bat for project management

Design doc: docs/plan/webdav-sync-design.md

scripts/start.bat

@@ -0,0 +1,53 @@
+@echo off
+chcp 65001 >nul 2>&1
+title 爱莉希雅待办事项
+
+echo ====================================================
+echo   爱莉希雅待办事项 - 启动脚本
+echo ====================================================
+echo.
+
+:: 项目根目录（脚本所在目录的上级）
+set "PROJECT_ROOT=%~dp0.."
+cd /d "%PROJECT_ROOT%"
+
+:: 检查 Python
+where python >nul 2>&1
+if errorlevel 1 (
+    echo [错误] 未找到 Python，请确保已安装并添加到 PATH
+    pause
+    exit /b 1
+)
+
+:: 检查依赖
+if not exist "api\app\__init__.py" (
+    echo [错误] 未找到项目文件，请确认在项目根目录运行
+    pause
+    exit /b 1
+)
+
+:: 安装 Python 依赖（如需）
+if not exist "api\__pycache__" (
+    echo [信息] 首次运行，安装 Python 依赖...
+    pip install -r requirements.txt -q
+    if errorlevel 1 (
+        echo [错误] Python 依赖安装失败
+        pause
+        exit /b 1
+    )
+)
+
+:: 检查端口占用
+echo [信息] 检查端口 23994 占用情况...
+for /f "tokens=5" %%a in ('netstat -ano -p TCP ^| findstr ":23994.*LISTENING"') do (
+    echo [警告] 端口 23994 已被进程 %%a 占用，正在尝试终止...
+    taskkill /PID %%a /F >nul 2>&1
+    timeout /t 2 /nobreak >nul
+)
+
+:: 启动项目
+echo [信息] 正在启动项目...
+echo [信息] 访问地址: http://localhost:23994
+echo.
+python main.py
+pause