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_thumbnails.py

@@ -1,6 +1,5 @@
 import pytest
 
-from backend.app import create_app
 from backend.app.celery_app import celery
 
 pytestmark = pytest.mark.integration
@@ -13,17 +12,6 @@ def eager():
     celery.conf.task_always_eager = False
 
 
-@pytest.fixture
-async def app():
-    return create_app()
-
-
-@pytest.fixture
-async def client(app):
-    async with app.test_client() as c:
-        yield c
-
-
 @pytest.mark.asyncio
 async def test_trigger_thumbnail_backfill(client):
     r = await client.post("/api/thumbnails/backfill")