feat(gaming): look up a public game by slug

[okapitula]

Summary

Adds GET /gaming/games/by-slug/{slug}, a public lookup that returns the same GameSchema payload as GET /gaming/games/{id}.

Why

A game detail page is addressed by its slug, not its UUID. Before this, a frontend had to list games and filter them client-side to resolve a slug. The new route applies the same playability filter as the id route: an inactive or unplayable game returns 404, and a malformed slug returns 400.

Alternatives considered

• A slug filter on the list route. Rejected: the list route is paginated and returns an array, so a detail page would have to unwrap it and handle an empty page itself.

Risks

None. This is an additive route. getGame now shares its lookup logic with getGameBySlug through a private findGame, and its behaviour is unchanged.