PC3 Phase 3.3 fix: RATE_LIMIT_ENABLED flag so the smoke suite isn't throttled

Every checkpoint smoke test registers users against one backend from one
client IP, so the per-IP auth limit tripped mid-suite (main_menu_full_flow
-> 2/12). Add RATE_LIMIT_ENABLED (default true); run_checkpoint_tests.py
sets it false for its backend. test_rate_limit.py forces it on in-fixture
so the limiter is still really tested.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

backend/tests/test_rate_limit.py

@@ -4,16 +4,21 @@ import pytest
 from fastapi.testclient import TestClient
 
 from app import rate_limit
+from app.config import settings
 from app.main import app
 
 
 @pytest.fixture(autouse=True)
 def _clean_limiter():
-    # This module tests the limiter itself, so start each test from empty
-    # and don't let conftest's between-test reset interfere mid-test.
+    # This module tests the limiter itself: force it on (the checkpoint
+    # runner disables it suite-wide via RATE_LIMIT_ENABLED) and start each
+    # test from an empty window.
+    prev = settings.RATE_LIMIT_ENABLED
+    settings.RATE_LIMIT_ENABLED = True
     rate_limit.reset_all()
     yield
     rate_limit.reset_all()
+    settings.RATE_LIMIT_ENABLED = prev
 
 
 def test_login_is_limited_after_the_threshold():