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

scripts/run_checkpoint_tests.py

@@ -90,6 +90,10 @@ def main():
     db_url = f"postgresql+asyncpg://agp_user:agp_password@localhost:{args.postgres_port}/agp_db"
     backend_env = os.environ.copy()
     backend_env["DATABASE_URL"] = db_url
+    # Every smoke test registers users against this one backend from one
+    # client IP; the per-IP auth rate limit would trip mid-suite.
+    # test_rate_limit.py exercises the limiter itself regardless.
+    backend_env["RATE_LIMIT_ENABLED"] = "false"
 
     print("\n=== Backend: alembic upgrade head ===")
     r = run([sys.executable, "-m", "alembic", "upgrade", "head"], cwd=BACKEND, env=backend_env)