Make auth tests use configurable admin password

- Add get_admin_password() and get_admin_username() helpers to conftest.py
- Update test_auth_api.py to use helpers instead of hardcoded credentials
- Allows tests to work with different passwords in feature/stage/prod envs

The ORCHARD_TEST_PASSWORD environment variable is set by CI jobs to match
the deployed environment's admin password.

backend/tests/conftest.py

@@ -56,6 +56,26 @@ os.environ.setdefault("ORCHARD_S3_BUCKET", "test-bucket")
 os.environ.setdefault("ORCHARD_S3_ACCESS_KEY_ID", "test")
 os.environ.setdefault("ORCHARD_S3_SECRET_ACCESS_KEY", "test")
 
+
+# =============================================================================
+# Admin Credentials Helper
+# =============================================================================
+
+
+def get_admin_password() -> str:
+    """Get the admin password for test authentication.
+
+    Returns the password from ORCHARD_TEST_PASSWORD environment variable,
+    or 'changeme123' as the default for local development.
+    """
+    return os.environ.get("ORCHARD_TEST_PASSWORD", "changeme123")
+
+
+def get_admin_username() -> str:
+    """Get the admin username for test authentication."""
+    return os.environ.get("ORCHARD_TEST_USERNAME", "admin")
+
+
 # Re-export factory functions for backward compatibility
 from tests.factories import (
     create_test_file,