refactor(tests): one definition each for the copied fixtures and fakes (#2825, milestone 296 area 1)

The shape ledger showed the same test scaffolding defined over and over:
_bind_user x12 (byte-identical), _dispose_engine x10 in three wordings,
_no_supersession x3, _make_mock_session x7 in three subsets, a get-or-create
User helper x2 (+3 inlined), and fifteen hand-rolled MagicMock note factories
each re-explaining the same "an auto-MagicMock attribute is truthy" hazard
(note 2109).

Now: conftest.py carries _bind_user / _dispose_engine / _no_supersession as
opt-in fixtures (pytestmark = usefixtures(...) per module, so unit tests pay
nothing), and tests/helpers.py carries make_mock_session(), ensure_user() and
fake_note(**attrs) — the hazard documented once, real values on every
attribute the product reads. Call sites were rewritten by AST so titles with
dashes and commas survived; the three SimpleNamespace _note stand-ins that
only feed a single function stay local.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_integration_db_maintenance.py

@@ -7,29 +7,14 @@ connection path that unit mocks cannot: the un-awaited
 AttributeError, reporting 0/6) passes the unit suite but fails here.
 """
 import pytest
-import pytest_asyncio
 
-from scribe.models import engine
 from scribe.services.db_maintenance import (
     MAINTENANCE_TABLES,
     get_table_health,
     run_maintenance,
 )
 
-pytestmark = pytest.mark.integration
-
-
-@pytest_asyncio.fixture(autouse=True)
-async def _dispose_engine():
-    """Dispose the app's module-level engine after each test.
-
-    The engine pools asyncpg connections per event loop, but pytest-asyncio runs
-    each test on a fresh loop — so without this, test 2 gets handed test 1's
-    connection bound to a now-dead loop ("Future attached to a different loop").
-    Disposing in the test's own loop teardown clears the pool cleanly.
-    """
-    yield
-    await engine.dispose()
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
 
 
 @pytest.mark.asyncio