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/s3_fs.rs

@@ -193,6 +193,13 @@ impl S3Vfs {
 }
 
 impl VfsBackend for S3Vfs {
+    fn clone_boxed(&self) -> Box<dyn VfsBackend> {
+        Box::new(Self {
+            bucket: self.bucket.clone(),
+            credentials: self.credentials.clone(),
+        })
+    }
+
     fn read_dir(&self, path: &Path) -> Result<Vec<VfsDirEntry>, VfsError> {
         let prefix = Self::dir_key(path);
         let list = self.list_objects(&prefix)?;