WebDAV improvements: flush fix, RwLock recovery, expired lock cleanup, atomic set_times

P0 fixes:
- flush(): add flushed flag, proper error logging, Drop warning for data loss
- props_data RwLock: replace unwrap() with try_read/try_write recovery
- PersistedLs: add is_expired() + cleanup_expired_locks() helper

P1 improvements:
- Props persistence via VFS (load_props/save_props/patch_props)
- COPY/MOVE sync dead props (copy on COPY, move key on rename)
- Atomic set_atime/set_mtime via filetime crate (no race condition)

New files:
- webdav_locks.rs: PersistedLs with lock persistence + expiry cleanup

Tests: 288 passed, 0 failed

markbase-core/src/lib.rs

@@ -23,6 +23,7 @@ pub mod ssh_server;
 pub mod sync;
 pub mod vfs;
 pub mod webdav;
+pub mod webdav_locks;
 pub mod webdav_version;
 
 #[cfg(test)]