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/conftest.py

@@ -54,7 +54,7 @@ def migrated_postgres_url(postgres_url: str) -> str:
     return postgres_url
 
 
-@pytest_asyncio.fixture(scope="session")
+@pytest_asyncio.fixture(scope="session", loop_scope="session")
 async def postgres_engine(migrated_postgres_url: str) -> AsyncIterator[AsyncEngine]:
     engine = create_engine(_settings_from_url(migrated_postgres_url))
     try:
@@ -63,18 +63,36 @@ async def postgres_engine(migrated_postgres_url: str) -> AsyncIterator[AsyncEngi
         await engine.dispose()
 
 
-@pytest_asyncio.fixture
-async def db_session(postgres_engine: AsyncEngine) -> AsyncIterator[AsyncSession]:
-    """One session per test, bound to a rolled-back outer transaction.
+@pytest_asyncio.fixture(loop_scope="session")
+async def db_sessionmaker(
+    postgres_engine: AsyncEngine,
+) -> AsyncIterator[async_sessionmaker[AsyncSession]]:
+    """A session *factory* per test, bound to a rolled-back outer transaction.
 
-    Isolates each test's writes (ADR-0016: isolate data per test) without
-    needing a fresh container or unique keys per test.
+    Every session it produces shares one connection/outer transaction, so
+    writes `commit()`ed by one session are visible to the next -- needed for
+    code under test that opens more than one session per operation (auth
+    resolution, the ADR-0017 two-phase upload) -- while the whole test's
+    writes still roll back together at teardown (ADR-0016: isolate data per
+    test).
     """
     async with postgres_engine.connect() as connection:
         outer_transaction = await connection.begin()
         sessionmaker = async_sessionmaker(
             bind=connection, expire_on_commit=False, join_transaction_mode="create_savepoint"
         )
-        async with sessionmaker() as session:
-            yield session
+        yield sessionmaker
         await outer_transaction.rollback()
+
+
+@pytest_asyncio.fixture(loop_scope="session")
+async def db_session(
+    db_sessionmaker: async_sessionmaker[AsyncSession],
+) -> AsyncIterator[AsyncSession]:
+    """One session per test, bound to a rolled-back outer transaction.
+
+    Isolates each test's writes (ADR-0016: isolate data per test) without
+    needing a fresh container or unique keys per test.
+    """
+    async with db_sessionmaker() as session:
+        yield session