(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/tests/test_sql_retention.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import json
+import stat
 from datetime import datetime, timezone
 from io import StringIO
 from pathlib import Path
@@ -85,6 +86,42 @@ class SqlRetentionTests(TestCase):
             self.assertFalse(SnapshotRecord.objects.filter(pk=old.pk).exists())
             self.assertEqual(result["deleted"], [{"dirname": old.dirname, "kind": "scheduled", "path": str(old_dir)}])
 
+    def test_apply_deletes_snapshot_with_readonly_data_directory(self) -> None:
+        with TemporaryDirectory() as tmp:
+            prefix = Path(tmp) / "home"
+            host = HostConfig.objects.create(
+                host="web-01",
+                address="web-01.example.test",
+                retention_daily=0,
+                retention_weekly=0,
+                retention_monthly=0,
+                retention_yearly=0,
+            )
+            old_dir = Path(tmp) / "backups" / host.host / "scheduled" / "20260518-021500Z__OLD"
+            old_data = old_dir / "data"
+            old_data.mkdir(parents=True)
+            old_data.joinpath("etc").mkdir()
+            old_data.joinpath("etc", "config").write_text("preserved permissions\n")
+            old_data.chmod(stat.S_IREAD | stat.S_IEXEC | stat.S_IRGRP | stat.S_IXGRP | stat.S_IROTH | stat.S_IXOTH)
+            new_dir = Path(tmp) / "backups" / host.host / "scheduled" / "20260519-021500Z__NEW"
+            new_dir.mkdir(parents=True)
+            old = self._snapshot(host, old_dir.name, path=str(old_data))
+            self._snapshot(host, new_dir.name, path=str(new_dir))
+
+            result = run_sql_retention_apply(
+                prefix=prefix,
+                host=host.host,
+                kind="scheduled",
+                protect_bases=False,
+                yes=True,
+                max_delete=1,
+                acquire_lock=False,
+            )
+
+            self.assertFalse(old_dir.exists())
+            self.assertFalse(SnapshotRecord.objects.filter(pk=old.pk).exists())
+            self.assertEqual(result["deleted"], [{"dirname": old.dirname, "kind": "scheduled", "path": str(old_dir)}])
+
     def test_apply_respects_max_delete(self) -> None:
         host = HostConfig.objects.create(
             host="web-01",