test: lock that the deck list omits cards

[lesnik512]

Why

PR #30 split the Deck response contract — light Deck for list/create/update, DeckWithCards for get_deck — but no test asserted that cards is absent from list responses. The existing tests passed under both the old and new schema, so the contract wasn't pinned.

What

test_list_decks_omits_cards creates a deck with a card, then asserts the list item has no cards key:

async def test_list_decks_omits_cards(client: AsyncClient) -> None:
    deck = await factories.DeckModelFactory.create_async()
    await factories.CardModelFactory.create_async(deck_id=deck.id)

    response = await client.get("/api/decks/")
    item = response.json()["items"][0]
    assert "cards" not in item

Under the old single-schema design this would have failed — list_decks returned the misleading cards: [] even for decks with cards (the noload lie #30 removed). So the test locks the fix and would catch a regression that re-added cards to the light contract. test_get_one_deck already locks the other half (detail includes cards).

21 passed, 100% coverage.

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