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_install.py

@@ -0,0 +1,20 @@
+"""mattermost-lts — INSTALL overlay (Phase 1d, DG4): override + extend-by-composition.
+Reuses the generic "really serving" assertion, then ADDS a recipe-specific check: mattermost's REST
+liveness endpoint /api/v4/system/ping answers 200 over real HTTPS through Traefik (proves the app +
+its DB-backed API are up, not just a proxy 200). Assertion-only on the shared deployment."""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from harness import generic, lifecycle  # noqa: E402
+
+
+def test_serving_and_api(live_app, meta):
+    # extend-by-composition: reuse the generic "really serving" assertion first ...
+    generic.assert_serving(live_app, meta)
+    # ... then the recipe-specific assertion: the mattermost REST liveness endpoint answers 200.
+    status = lifecycle.http_get(live_app, "/api/v4/system/ping")
+    assert status == 200, (
+        f"expected 200 from {live_app}/api/v4/system/ping, got {status}"
+    )