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

Cargo.lock

@@ -2675,6 +2675,7 @@ dependencies = [
  "base64",
  "bcrypt",
  "byteorder",
+ "bytes",
  "chrono",
  "cipher 0.4.4",
  "clap",
@@ -2689,7 +2690,6 @@ dependencies = [
  "futures-util",
  "hmac 0.12.1",
  "log",
- "markbase-webdav",
  "md5 0.8.0",
  "nix 0.29.0",
  "postgres",