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

Cargo.lock

@@ -2890,6 +2890,7 @@ dependencies = [
  "futures-util",
  "hex",
  "hmac 0.12.1",
+ "http",
  "log",
  "md5 0.8.0",
  "nix 0.29.0",
@@ -2926,6 +2927,7 @@ dependencies = [
  "url",
  "uuid",
  "x25519-dalek",
+ "xmltree",
  "xz2",
  "zip",
  "zstd 0.13.3",