[Backend] Expose admin customer and profile management APIs

[juliuskrah]

Related admin initiative: #287
Blocks admin issues: #298, #301

Summary

Expose backend APIs/read models that let the commerce admin inspect and manage customer and profile support data.

Acceptance criteria

• gitstore-api exposes admin-scoped list and detail queries for customer/profile records where those resources exist.
• Responses include namespace scope, identity references, profile fields, session/order relationships, and redacted sensitive data.
• The contract explicitly defines which customer/profile fields or support actions are editable, read-only, or unavailable.
• Authorization, missing-profile, cross-namespace, and unsupported-operation cases return structured errors for admin UI mapping.
• Tests cover search/list/detail, relationship loading, namespace isolation, authorization failures, and every supported mutation or read-only unsupported response.
• Identity-provider ownership remains outside GitStore unless a field is explicitly documented as a GitStore-owned projection.

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.