feat(2): mattermost-lts P4 data-integrity overlay (ops.py postgres ci_marker seed + test_install/upgrade/backup/restore) — verifying recipe's PGDATA-dir restore brings the marker back

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/mattermost-lts/test_backup.py

@@ -0,0 +1,23 @@
+"""mattermost-lts — BACKUP overlay (Phase 1e HC3): assertion-only + additive.
+ops.pre_backup wrote "original" into postgres before the backup op (the recipe's db pre-hook dumps
+the DB + archives the PGDATA dir). This overlay ADDS: the seeded row is intact at backup time."""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from harness import lifecycle  # noqa: E402
+
+
+def _psql(domain, sql):
+    cmd = (
+        "PGPASSWORD=$(cat /run/secrets/postgres_password) "
+        f'psql -U mattermost -d mattermost -tAc "{sql}"'
+    )
+    return lifecycle.exec_in_app(domain, ["sh", "-c", cmd], service="db").strip()
+
+
+def test_backup_captures_state(live_app):
+    assert (
+        _psql(live_app, "SELECT v FROM ci_marker;") == "original"
+    ), "the seeded postgres state was not present at backup time"