PC3 Phase 3.3: rate-limit the auth endpoints

app/rate_limit.py: an in-process sliding-window limiter (single-worker
backend, so no Redis needed -- flagged for if that changes). /register and
/login are capped at 10 attempts / minute / client IP (X-Forwarded-For
aware), returning 429 + Retry-After past that. Blunts credential
brute-force, which the backend had no protection against.

- conftest.py resets the limiter between tests (every TestClient request
  looks like one IP).
- tests/test_rate_limit.py: threshold, shared bucket, per-key window. 77 passed.

CORS reviewed while here: already an explicit origin allowlist
(settings.CORS_ORIGINS), not "*" -- and the Godot client isn't a browser
so it doesn't apply there anyway. Left as-is.

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

backend/tests/conftest.py

@@ -132,6 +132,17 @@ def _reset_database():
     yield
 
 
+@pytest.fixture(autouse=True)
+def _reset_rate_limiter():
+    """Every TestClient request looks like the same client IP, so the
+    per-IP auth limiter would trip mid-suite. Clear it between tests; the
+    dedicated rate-limit test disables this to exercise the real behaviour.
+    """
+    from app import rate_limit
+    rate_limit.reset_all()
+    yield
+
+
 @pytest.fixture(autouse=True)
 def _override_db():
     """Override get_db so the app uses a fresh session per test.