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_cleanup.py

@@ -11,7 +11,6 @@ import pytest
 from PIL import Image
 from sqlalchemy import func, select
 
-from backend.app import create_app
 from backend.app.celery_app import celery
 from backend.app.models import ImageRecord, LibraryAuditRun
 
@@ -23,17 +22,6 @@ def disable_celery_eager(monkeypatch):
     monkeypatch.setattr(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
-
-
 def _sha256_min_dim_token(min_w: int, min_h: int) -> str:
     canon = f"{min_w}x{min_h}"
     return f"delete-min-dim-{hashlib.sha256(canon.encode()).hexdigest()[:8]}"