Fix Studio Tests CI: async event loop error in backend auth tests

[Copilot]

Studio Tests / Backend failing on both Python 3.10 and 3.11 with RuntimeError: There is no current event loop in thread 'MainThread' — all 4 TestAuthEndpoints tests erroring out.

Root cause: the client fixture used asyncio.get_event_loop().run_until_complete() to set up the DB, which has been broken since Python 3.10 deprecated implicit event loop creation.

Changes

• web_strategy_studio/backend/tests/test_auth.py — Convert client from a sync fixture to an async fixture; replace get_event_loop().run_until_complete() with direct await; use async with AsyncClient(...) for proper lifecycle; dispose engine after yield
• web_strategy_studio/backend/pyproject.toml — Add asyncio_mode = "auto" to pytest config so async fixtures are handled without per-fixture decoration

# Before (broken on 3.10+)
@pytest.fixture
def client(self, tmp_path):
    asyncio.get_event_loop().run_until_complete(setup_db())
    return AsyncClient(...)

# After
@pytest.fixture
async def client(self, tmp_path):
    async with new_engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    async with AsyncClient(...) as ac:
        yield ac
    await new_engine.dispose()

All 141 backend tests pass.