feat: complete auth/profile username migration and runtime safeguards

backend/tests/unit/database/test_profile_migration_contract.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+
+def test_drop_display_name_migration_exists_and_uses_username_metadata() -> None:
+    versions_dir = Path(__file__).resolve().parents[3] / "alembic" / "versions"
+    migration = (
+        versions_dir / "20260224_drop_profile_display_name_and_trigger_username.py"
+    )
+
+    assert migration.exists()
+
+    content = migration.read_text(encoding="utf-8")
+    assert "DROP COLUMN" in content and "display_name" in content
+    assert "raw_user_meta_data->>'username'" in content