[Backend] Expose admin order, checkout, and basket operation APIs

[juliuskrah]

Related admin initiative: #287
Blocks admin issues: #297, #301

Summary

Expose backend APIs/read models that let the commerce admin inspect and support order, checkout, and basket operations.

Acceptance criteria

• gitstore-api exposes admin-scoped list and detail queries for baskets, checkout sessions, and orders where those resources exist.
• Responses include namespace scope, customer/session references, line items, ProductVariant references, quantities, price snapshots, status, timestamps, and backend-owned identifiers.
• The contract explicitly defines which support actions are available, unavailable, or read-only for baskets, checkout sessions, and orders.
• Loading, not-found, unauthorized, stale, and unsupported-operation cases return structured errors that admin views can map directly.
• Tests cover list/detail queries, namespace isolation, authorization failures, and every supported operation or read-only unsupported response.
• The admin contract remains compatible with the storefront cart, checkout, and order-status APIs without exposing storefront-only assumptions.

Architecture note

The commerce admin remains an optional add-on and consumes gitstore-api; it must not talk directly to gitstore-git-service. In admin workflows, a namespace is the store context.