feat: add cartLoader to batch User.cart resolution

[pozylon]

Problem

The cart query in core-orders (configureOrdersModule-queries.ts) used a plain findOne with no batching. Resolving the cart field across multiple users in a single GraphQL request (e.g. admin users { cart { ... } }) issued one DB query per user.

Change

Adds a request-scoped DataLoader for carts, keyed by { userId, countryCode } (plus orderNumber), following the existing loader conventions in packages/api/src/loaders.

• core-orders: extract a shared buildCartSelector helper and add a batched findCarts({ userIds, ... }) query (userId: { $in }, sorted updated: -1). cart now reuses the helper (behavior unchanged — every caller already supplies countryCode). The loader lives in the API layer and can't query MongoDB directly (layer boundary), and findOrders/buildFindSelector only support a single userId, so a dedicated batched method was needed.
• cartLoader.ts (new): batches all keys into one findCarts({ userIds }) query, then resolves each key in JS by matching userId + countryCode + orderNumber against the most-recently-updated-first list — mirroring modules.orders.cart semantics.
• User.cart resolver: now calls loaders.cartLoader.load(...), covering me { cart }, user(userId) { cart }, and users { cart }.

Intentionally left on the direct modules.orders.cart call

• Core services (nextUserCart, migrateOrderCart) and platform/worker — no request-scoped loaders there.
• signPaymentProviderForCheckout mutation — avoids loader-cache staleness in the write path.
• getUserCart MCP tool — single read, no batching benefit.

Verification

• npm run build (full typecheck) passes; ESLint clean on changed files.
• 88 integration tests pass: heartbeat, cart-products, cart-migrate, auth-admin (all select me/cross-user cart), plus cart-checkout, cart-delivery-payment, cart-discounts, cart-quotations (exercise the refactored cart/findCarts path via checkout services).

[pozylon]

Closing — this change should target the v4.8.x branch, not master. Re-doing it on a branch based off v4.8.x.