(bugfix) Prune snapshots with preserved read-only permissions

Make SQL retention delete the snapshot root when records point at the
snapshot data directory, matching how backup metadata is stored on disk.

Before removing a snapshot tree, temporarily add user write permission to
directories inside that snapshot so rsync-preserved source permissions do
not block cleanup.

Add a regression test for pruning snapshots whose data directory mirrors a
read-only remote root.

src/pobsync_backend/retention.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import shutil
+import stat
 from datetime import datetime, timezone
 from pathlib import Path
 from typing import Any
@@ -89,17 +90,17 @@ def run_sql_retention_apply(
             if snap_kind not in {"scheduled", "manual"}:
                 raise ConfigError(f"Refusing to delete unsupported snapshot kind: {snap_kind!r}")
 
-            path = Path(snap_path)
+            path = _snapshot_delete_path(path=Path(snap_path), dirname=dirname)
             if not path.exists():
                 actions.append(f"skip missing {snap_kind}/{dirname}")
                 continue
             if not path.is_dir():
-                raise ConfigError(f"Refusing to delete non-directory path: {snap_path}")
+                raise ConfigError(f"Refusing to delete non-directory path: {path}")
 
-            shutil.rmtree(path)
+            _remove_snapshot_tree(path)
             SnapshotRecord.objects.filter(host__host=host, kind=snap_kind, dirname=dirname).delete()
             actions.append(f"deleted {snap_kind} {dirname}")
-            deleted.append({"dirname": dirname, "kind": snap_kind, "path": snap_path})
+            deleted.append({"dirname": dirname, "kind": snap_kind, "path": str(path)})
 
         return {
             "ok": True,
@@ -180,3 +181,22 @@ def _snapshot_to_delete_item(snapshot: Snapshot) -> dict[str, Any]:
         "dt": snapshot.dt.isoformat(),
         "status": snapshot.status,
     }
+
+
+def _snapshot_delete_path(*, path: Path, dirname: str) -> Path:
+    if path.name == "data" and path.parent.name == dirname:
+        return path.parent
+    return path
+
+
+def _remove_snapshot_tree(path: Path) -> None:
+    _make_directories_user_writable(path)
+    shutil.rmtree(path)
+
+
+def _make_directories_user_writable(path: Path) -> None:
+    for directory in [path, *[child for child in path.rglob("*") if child.is_dir() and not child.is_symlink()]]:
+        mode = directory.stat().st_mode
+        if mode & stat.S_IWUSR:
+            continue
+        directory.chmod(mode | stat.S_IWUSR)