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/task.py

@@ -1,3 +1,4 @@
+import uuid as _uuid
 from sqlalchemy import Column, Integer, String, Text, Boolean, DateTime, ForeignKey
 from sqlalchemy.orm import relationship
 from app.database import Base
@@ -9,12 +10,15 @@ class Task(Base):
     __tablename__ = "tasks"
 
     id = Column(Integer, primary_key=True, index=True)
+    uuid = Column(String(36), default=lambda: str(_uuid.uuid4()), unique=True, index=True)
     title = Column(String(200), nullable=False)
     description = Column(Text, nullable=True)
     priority = Column(String(20), default="q4")  # q1(重要紧急), q2(重要不紧急), q3(不重要紧急), q4(不重要不紧急)
     due_date = Column(DateTime, nullable=True)
     is_completed = Column(Boolean, default=False)
+    is_deleted = Column(Boolean, default=False)
     category_id = Column(Integer, ForeignKey("categories.id"), nullable=True)
+    sync_version = Column(Integer, default=1)
     created_at = Column(DateTime, default=utcnow)
     updated_at = Column(DateTime, default=utcnow, onupdate=utcnow)