fix(api): enforce the listing price rule on update

[SKonteye]

What

• ListingsService.update now applies the same price rule as create, based on the stored listing type (the type cannot change after publication):
  • a sale rejects price: null with 400 A price is required for a sale;
  • a donation, barter or request ignores price, as create already does.
• assertOwnership returns the listing type so the rule needs no extra query.
• New unit spec listings.service.spec.ts (ownership check, price rule on a sale, price ignored on a donation).
• Two end-to-end cases in listings.e2e-spec.ts for the same rule over HTTP.

Why

create enforces the rule, update did not. @IsOptional() in UpdateListingDto lets an explicit null through validation, so PATCH /listings/:id with { "price": null } turned a sale into a listing with no price, and { "price": 3000 } on a donation was persisted and shown publicly. This is one of the money-adjacent rules CONTRIBUTING asks to cover with tests.

Testing

• pnpm lint
• pnpm typecheck
• pnpm test (11 unit tests, 4 new)
• pnpm --filter @wantere/api test:e2e against the docker Postgres and Redis (11 tests, 2 new)
• pnpm build
• The two new end-to-end cases fail on main without the service change and pass with it.

No API surface change, so pnpm api:generate was not needed.

Screenshots

N/A, API only.

https://claude.ai/code/session_01TwmkH7BorySLrvJkMGGo3G

[Denver-sn]

La règle de prix est maintenant appliquée sur la mise à jour en s'appuyant sur le type stocké après contrôle de propriété : les ventes conservent leur prix si le champ est absent, refusent null et acceptent une nouvelle valeur ; les autres types ignorent price. Les tests unitaires et E2E couvrent ces chemins, ainsi que l'autorisation. Aucun finding introduit par ce diff.

Vérification locale non exécutée : pnpm/corepack n'est pas disponible dans l'environnement de revue ; les checks GitHub affichés ne contiennent aucun résultat.