Fix CI integration test rate limiting

- Add auth_intensive marker for tests that make many login requests
- Mark all tests in test_auth_api.py with auth_intensive
- Exclude auth_intensive tests from CI integration runs against deployed
  environments (they trigger 429 rate limiting)
- Remove duplicate TestSecurityEdgeCases class definition
- Register auth_intensive, integration, large, slow markers in conftest.py

backend/tests/conftest.py

@@ -9,6 +9,33 @@ This module provides:
 
 import os
 import pytest
+
+
+# =============================================================================
+# Pytest Markers
+# =============================================================================
+
+
+def pytest_configure(config):
+    """Register custom pytest markers."""
+    config.addinivalue_line(
+        "markers",
+        "auth_intensive: marks tests that make many login requests (excluded from CI integration tests due to rate limiting)",
+    )
+    config.addinivalue_line(
+        "markers",
+        "integration: marks tests as integration tests",
+    )
+    config.addinivalue_line(
+        "markers",
+        "large: marks tests that handle large files (slow)",
+    )
+    config.addinivalue_line(
+        "markers",
+        "slow: marks tests as slow running",
+    )
+
+
 import io
 from typing import Generator
 from unittest.mock import MagicMock