feat(auth): add rate limiting to login and register endpoints

- Add slowapi rate limiter: 10 req/min for /auth/login, 5 req/min for /auth/register
- Return HTTP 429 with Retry-After header when limit is exceeded
- Add slowapi to requirements.txt
- Add 4 passing tests for rate limit behavior

Closes leeworks-agents/SPARC#9

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_rate_limit.py

@@ -0,0 +1,97 @@
+"""Tests for rate limiting on auth endpoints."""
+
+import pytest
+from unittest.mock import Mock, patch, MagicMock
+from fastapi.testclient import TestClient
+
+from SPARC.api import app
+
+
+@pytest.fixture
+def client():
+    """Create test client with rate limiter enabled."""
+    return TestClient(app)
+
+
+@pytest.fixture(autouse=True)
+def reset_limiter():
+    """Reset rate limiter storage between tests."""
+    from SPARC.api import limiter
+    limiter.reset()
+    yield
+
+
+class TestRateLimiting:
+    """Test rate limiting on login and register endpoints."""
+
+    @patch("SPARC.api.get_db_client")
+    def test_login_allows_requests_under_limit(self, mock_db_client, client):
+        """Login endpoint allows requests under the rate limit."""
+        mock_db = MagicMock()
+        mock_db.authenticate_user.return_value = None
+        mock_db_client.return_value = mock_db
+
+        # Should allow at least a few requests
+        for _ in range(5):
+            response = client.post(
+                "/auth/login",
+                json={"email": "test@example.com", "password": "password123"},
+            )
+            # 401 is expected (invalid credentials), not 429
+            assert response.status_code == 401
+
+    @patch("SPARC.api.get_db_client")
+    def test_login_rate_limited_after_threshold(self, mock_db_client, client):
+        """Login endpoint returns 429 after exceeding rate limit."""
+        mock_db = MagicMock()
+        mock_db.authenticate_user.return_value = None
+        mock_db_client.return_value = mock_db
+
+        # Send more than the limit (10/minute)
+        statuses = []
+        for _ in range(15):
+            response = client.post(
+                "/auth/login",
+                json={"email": "test@example.com", "password": "password123"},
+            )
+            statuses.append(response.status_code)
+
+        # At least one should be 429
+        assert 429 in statuses, f"Expected 429 in statuses but got: {set(statuses)}"
+
+    @patch("SPARC.api.get_db_client")
+    def test_register_rate_limited_after_threshold(self, mock_db_client, client):
+        """Register endpoint returns 429 after exceeding rate limit."""
+        mock_db = MagicMock()
+        mock_db.get_user_count.return_value = 1
+        mock_db.create_user.return_value = None  # triggers 400 (email exists)
+        mock_db_client.return_value = mock_db
+
+        # Send more than the limit (5/minute)
+        statuses = []
+        for _ in range(10):
+            response = client.post(
+                "/auth/register",
+                json={"email": "test@example.com", "password": "password123"},
+            )
+            statuses.append(response.status_code)
+
+        # At least one should be 429
+        assert 429 in statuses, f"Expected 429 in statuses but got: {set(statuses)}"
+
+    @patch("SPARC.api.get_db_client")
+    def test_rate_limit_returns_retry_after_header(self, mock_db_client, client):
+        """Rate limited responses include a Retry-After header."""
+        mock_db = MagicMock()
+        mock_db.authenticate_user.return_value = None
+        mock_db_client.return_value = mock_db
+
+        # Exhaust the limit
+        for _ in range(15):
+            response = client.post(
+                "/auth/login",
+                json={"email": "test@example.com", "password": "password123"},
+            )
+            if response.status_code == 429:
+                assert "Retry-After" in response.headers
+                break