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/schemas/anniversary.py

@@ -54,6 +54,7 @@ class AnniversaryCategoryUpdate(BaseModel):
 class AnniversaryCategoryResponse(AnniversaryCategoryBase):
     """纪念日分类响应模型"""
     id: int
+    uuid: Optional[str] = None
 
     class Config:
         from_attributes = True
@@ -111,6 +112,7 @@ class AnniversaryUpdate(BaseModel):
 class AnniversaryResponse(AnniversaryBase):
     """纪念日响应模型"""
     id: int
+    uuid: Optional[str] = None
     created_at: datetime
     updated_at: datetime
     category: Optional[AnniversaryCategoryResponse] = None