test(postgres): support multi-session integration tests

Why:
- Code under test (auth resolution, the two-phase upload) opens more than
  one session per operation; the existing fixture only exposed one
  rolled-back session.

Changes:
- Add a db_sessionmaker fixture sharing one outer transaction.
- Pin loop_scope="session" -- without it, a second async test against the
  session-scoped Postgres container fails with "Event loop is closed."

tests/integration/postgres/test_migrations.py

@@ -2,7 +2,11 @@ import pytest
 from sqlalchemy import inspect
 from sqlalchemy.ext.asyncio import AsyncEngine
 
-pytestmark = [pytest.mark.integration, pytest.mark.postgres, pytest.mark.asyncio]
+pytestmark = [
+    pytest.mark.integration,
+    pytest.mark.postgres,
+    pytest.mark.asyncio(loop_scope="session"),
+]
 
 EXPECTED_TABLES = {
     "tenants",