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_downloads.py

@@ -1,22 +1,10 @@
 import pytest
 
-from backend.app import create_app
 from backend.app.models import Artist, DownloadEvent, Source
 
 pytestmark = pytest.mark.integration
 
 
-@pytest.fixture
-async def app():
-    return create_app()
-
-
-@pytest.fixture
-async def client(app):
-    async with app.test_client() as c:
-        yield c
-
-
 @pytest.fixture
 async def seed(db):
     artist = Artist(name="Alice", slug="alice")