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_upgrade.py

@@ -0,0 +1,23 @@
+"""mattermost-lts — UPGRADE overlay (Phase 1e HC3): data-continuity, assertion-only + additive.
+ops.pre_upgrade wrote a postgres marker row before the upgrade; this overlay ADDS: the postgres data
+survived the chaos crossover. Read via psql in the `db` service."""
+
+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_upgrade_preserves_data(live_app):
+    assert (
+        _psql(live_app, "SELECT v FROM ci_marker;") == "upgrade-survives"
+    ), "postgres data did not survive the upgrade"