fix(auth): enforce role boundaries on data quality and audit

The data-quality workbench (list, detail, defer, reject) and the audit trail
had no role gate at all beyond authentication -- confirmed live, a Rental
Employee session could list and resolve data-quality issues and read the
full audit trail through both the API and the UI, with only merge-customers
and scan already restricted.

Per the role matrix, both areas are Operations-Manager-only. Gate the
remaining data-quality and audit endpoints with require_operations_manager,
hide their nav items for Rental Employee, show the same restricted-message
pattern Automation.tsx already used for direct URL access, and stop the
dashboard from linking into now-restricted areas for that role.

backend/tests/test_audit.py

@@ -9,3 +9,8 @@ def test_demo_login_is_audited(ops_client):
 def test_audit_requires_authentication(client):
     response = client.get("/api/v1/audit")
     assert response.status_code == 401
+
+
+def test_audit_requires_operations_manager(employee_client):
+    response = employee_client.get("/api/v1/audit")
+    assert response.status_code == 403