[Backend] Define admin catalog write contract for GitStore resources

[juliuskrah]

Related admin initiative: #287
Blocks admin issues: #289, #292, #293, #300, #301

Summary

Define and implement the backend write contract that the commerce admin uses for catalog resources: Product, ProductVariant, Collection, and CategoryTaxonomy.

Acceptance criteria

• gitstore-api exposes a documented admin-safe write path for creating, updating, and deleting supported catalog resources, or explicitly documents any resource that remains read-only.
• The contract uses GitStore resource names, namespace scope, resourceVersion or equivalent conflict protection, and structured validation errors.
• Product and ProductVariant writes preserve the Product-as-aggregate and ProductVariant-as-sellable-unit model.
• Collection and CategoryTaxonomy writes preserve hierarchy, selector, and single-category constraints already enforced by the backend.
• Writes are persisted through the backend-owned Git/resource pipeline and never require the admin client to call gitstore-git-service directly.
• Integration tests cover successful writes, validation failures, conflict handling, namespace boundary checks, and generated GraphQL schema/codegen compatibility.

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.