feat: split initial social schema migration chain

replace monolithic migration with ordered scripts, include profiles/sessions in migration, and verify full downgrade/upgrade cycle for clean Supabase bootstrap

backend/tests/unit/database/test_profile_migration_contract.py

@@ -3,14 +3,13 @@ from __future__ import annotations
 from pathlib import Path
 
 
-def test_drop_display_name_migration_exists_and_uses_username_metadata() -> None:
+def test_profiles_has_settings_column() -> None:
+    """Verify profiles and auth trigger are defined in split migrations."""
     versions_dir = Path(__file__).resolve().parents[3] / "alembic" / "versions"
-    migration = (
-        versions_dir / "20260224_drop_profile_display_name_and_trigger_username.py"
-    )
+    migration_files = sorted(versions_dir.glob("20260226_*.py"))
+    content = "\n".join(m.read_text(encoding="utf-8") for m in migration_files)
 
-    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
+    assert 'create_table(\n        "profiles"' in content
+    assert '"settings"' in content
+    assert "create_profile_for_new_user" in content
+    assert "on_auth_user_created" in content