Implement WebDAV VFS integration: dav-server 0.11 compatible

- Add webdav.rs module: VfsDavFs, VfsDavFile, VfsDavMetaData
- Implement DavFileSystem + Clone for GuardedFileSystem blanket impl
- Add clone_boxed to VfsBackend trait (required for Sync)
- Update CLI webdav.rs to use VFS instead of SQLite
- Add bytes dependency
- All 155 tests pass

markbase-core/src/vfs/mod.rs

@@ -115,7 +115,10 @@ pub trait VfsFile {
 }
 
 /// VFS 后端 trait（所有文件系统操作）
-pub trait VfsBackend: Send {
+pub trait VfsBackend: Send + Sync {
+    /// Clone boxed
+    fn clone_boxed(&self) -> Box<dyn VfsBackend>;
+
     /// 读取目录内容
     fn read_dir(&self, path: &Path) -> Result<Vec<VfsDirEntry>, VfsError>;