refactor: split the Deck response contract from its detail view

CLAUDE.md

@@ -71,4 +71,5 @@ Endpoints inject repositories with `FromDI(Repository)` from `modern_di_fastapi`
 - Type-ignore syntax is `# ty: ignore[error-code]` (this project uses `ty`, not mypy). See `app/application.py:39` for an example.
 - Ruff is configured with `select = ["ALL"]` and a curated ignore list in `pyproject.toml`. Don't sprinkle `# noqa`; prefer fixing or extending the project ignore list if a rule is genuinely wrong for the codebase.
 - Routes convert ORM objects to schemas explicitly, never via `typing.cast`. Single objects: `schemas.X.model_validate(instance)`. Collections: `schemas.Xs.from_models(objects)` (the `Collection[T]` seam in `app/schemas.py`). Both rely on `from_attributes=True`.
+- Deck responses are deliberately two-shaped: `list_decks`/`create_deck`/`update_deck` return the light `schemas.Deck` (no `cards`), while `get_deck` returns `schemas.DeckWithCards`. The split mirrors loading — lists/writes use `noload` (no cards query), detail uses `selectinload` via `fetch_with_cards` — so the type states exactly what each endpoint loads.
 - Line length is 120.

app/api/decks.py

@@ -22,9 +22,9 @@ async def list_decks(
 async def get_deck(
     deck_id: int,
     decks_repository: DecksRepository = FromDI(DecksRepository),
-) -> schemas.Deck:
+) -> schemas.DeckWithCards:
     instance = await decks_repository.fetch_with_cards(deck_id)
-    return schemas.Deck.model_validate(instance)
+    return schemas.DeckWithCards.model_validate(instance)
 
 
 @ROUTER.put("/decks/{deck_id}/")

app/schemas.py

@@ -49,8 +49,15 @@ class DeckCreate(DeckBase):
 
 
 class Deck(DeckBase):
+    """Light deck view for lists and writes; cards are not loaded."""
+
     id: PositiveInt
-    cards: list[Card] | None
+
+
+class DeckWithCards(Deck):
+    """Deck detail view; cards are eager-loaded (selectinload) and always present."""
+
+    cards: list[Card]
 
 
 class Decks(Collection[Deck]):