feat(mailu): add ops.py + backup/restore tests + update PARITY.md (P4 now covered via PR#3)

tests/mailu/test_backup.py

@@ -0,0 +1,25 @@
+"""mailu — BACKUP overlay: assert the seeded mailbox is present at backup time.
+
+ops.pre_backup created citest@<domain> via the admin container; this overlay verifies the admin
+sqlite DB contains that user at the moment the backup is taken. The backup→restore divergence
+is in ops.pre_restore (which deletes the user before restore runs)."""
+
+from __future__ import annotations
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(__file__))
+import _mailu  # noqa: E402
+
+_CI_LOCALPART = "citest"
+
+
+def test_backup_captures_mailbox(live_app):
+    email = f"{_CI_LOCALPART}@{live_app}"
+    cfg = _mailu.config_export(live_app)
+    emails = _mailu.user_emails(cfg)
+    assert email in emails, (
+        f"seeded mailbox {email!r} not found in config-export at backup time; "
+        f"users present: {emails}"
+    )