test: add comprehensive integration test suite for IAM platform

Add 162 integration tests covering authentication flows, TOTP MFA,
SSH key/certificate management, organization workflows, multi-org
access, self-service features, admin operations, authorization,
security edge cases, department/principal management, CA management,
policy compliance, WebAuthn passkeys, and ZeroTier network access.

Includes:
- Reusable API client library with session management
- Test fixtures for users, organizations, memberships, and CAs
- Helper functions for SSH key generation and verification
- Documentation for running and writing tests

Also update test configuration to disable conflicting maas plugins
and configure WebAuthn/session settings for localhost testing.

pytest.ini

@@ -6,6 +6,9 @@ python_functions = test_*
 addopts =
     -v
     --strict-markers
+    -p no:maas-django
+    -p no:maas-perftest
+    -p no:maas-seeds
     --cov=gatehouse_app
     --cov-report=term-missing
     --cov-report=html