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

api/app/models/sync_settings.py

@@ -0,0 +1,28 @@
+import uuid as _uuid
+from sqlalchemy import Column, Integer, String, Boolean, DateTime
+from app.database import Base
+from app.utils.datetime import utcnow
+
+
+class SyncSettings(Base):
+    """同步设置模型（单例，始终只有一条记录 id=1）"""
+    __tablename__ = "sync_settings"
+
+    id = Column(Integer, primary_key=True, default=1)
+
+    # WebDAV 连接配置
+    webdav_url = Column(String(500), nullable=True)
+    webdav_username = Column(String(200), nullable=True)
+    webdav_password = Column(String(500), nullable=True)  # AES-256-GCM 加密存储
+    webdav_path = Column(String(200), default="/elysia-todo/")
+
+    # 同步状态
+    sync_enabled = Column(Boolean, default=False)
+    last_sync_at = Column(DateTime, nullable=True)
+    last_sync_version = Column(Integer, default=0)
+    auto_sync = Column(Boolean, default=False)
+    auto_sync_interval = Column(Integer, default=300)  # 秒
+
+    # 时间戳
+    created_at = Column(DateTime, default=utcnow)
+    updated_at = Column(DateTime, default=utcnow, onupdate=utcnow)