[Backend] Implement admin ProductVariant inventory and pricing operations

[juliuskrah]

Related admin initiative: #287
Blocks admin issues: #292, #296, #301

Summary

Define and implement backend operations needed by the commerce admin for ProductVariant inventory and pricing fields.

Acceptance criteria

• gitstore-api exposes documented operations or resource write semantics for changing ProductVariant inventory and pricing data.
• Validation covers numeric bounds, currency/price representation, managed inventory behavior, inventory policy, and ProductVariant identity.
• Conflict handling prevents overwriting newer variant changes without an explicit resourceVersion or equivalent precondition.
• Responses include structured success, validation, authorization, not-found, and conflict errors that admin forms can map directly.
• Integration tests cover memdb and ScyllaDB behavior where both backends support the affected data.
• Documentation explains which inventory and pricing fields are authoritative in ProductVariant spec or status.

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.