feat(demo): plain-language integration status, richer audit, reset integrity

Integration status badges across Dashboard/Automation now show honest
plain-language labels instead of raw backend state strings (and fix a
few states that had no matching CSS colour class at all). Audit trail
gets a "view related events" action reusing the existing correlation_id
filter. About page gains scope/architecture/security/testing sections
and a guided-demo entry point. POST /api/v1/demo/reset now runs and
records a server-side scenario-integrity check. Also fixes a second real
race condition (caught by the return-review e2e test): the odometer
scenario pre-fill now resolves before ReturnForm mounts instead of
patching its value in after the fact.

backend/tests/test_auth.py

@@ -17,9 +17,12 @@ def test_rental_employee_cannot_reset_demo(employee_client):
 def test_operations_manager_can_reset_demo(ops_client):
     response = ops_client.post("/api/v1/demo/reset")
     assert response.status_code == 200
-    assert response.json()["counts"]["vehicles"] == 50
-    assert response.json()["anchor_date"]
-    assert response.json()["seeded_at"]
+    body = response.json()
+    assert body["counts"]["vehicles"] == 50
+    assert body["anchor_date"]
+    assert body["seeded_at"]
+    assert body["scenario_integrity"]["all_ready"] is True
+    assert body["scenario_integrity"]["not_ready"] == []
 
 
 def test_reset_is_rejected_when_demo_allow_reset_is_disabled(ops_client, monkeypatch):