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/__init__.py

@@ -5,10 +5,12 @@ from app.models.user_settings import UserSettings
 from app.models.habit import HabitGroup, Habit, HabitCheckin
 from app.models.anniversary import AnniversaryCategory, Anniversary
 from app.models.goal import Goal, GoalStep, GoalReview, goal_tasks
+from app.models.sync_settings import SyncSettings
 
 __all__ = [
     "Task", "Category", "Tag", "task_tags", "UserSettings",
     "HabitGroup", "Habit", "HabitCheckin",
     "AnniversaryCategory", "Anniversary",
     "Goal", "GoalStep", "GoalReview", "goal_tasks",
+    "SyncSettings",
 ]