refactor(dry-S1): hoist app/client test fixtures into conftest

Removed the app/client fixtures duplicated across 36 test files (two
variants: separate app + client(app), and a self-contained client() that
called create_app inline) and the now-unused create_app imports. Both
fixtures now live once in conftest.py. test_suggestions_bulk keeps its
import (builds the app inline in two tests); test_health drops its local
client + unused pytest_asyncio.

Net -415 lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_api_migrate.py

@@ -6,7 +6,6 @@ import pytest
 from werkzeug.datastructures import FileStorage
 
 import backend.app.tasks.migration  # noqa: F401 — register celery task
-from backend.app import create_app
 from backend.app.celery_app import celery
 
 pytestmark = pytest.mark.integration
@@ -20,17 +19,6 @@ def _file_storage(payload: dict, filename: str = "export.json") -> FileStorage:
     )
 
 
-@pytest.fixture
-async def app():
-    return create_app()
-
-
-@pytest.fixture
-async def client(app):
-    async with app.test_client() as c:
-        yield c
-
-
 # Retired 2026-05-24 (FC-3h): `test_post_backup_returns_202_and_id`
 # asserted the /api/migrate/backup endpoint, which was retired in FC-3h.
 # Backup is now a first-class feature at /api/system/backup/*;