[Initiative] Datastore Backend: go-memdb

[juliuskrah]

Summary

Implement a go-memdb datastore backend for the new API datastore abstraction as an in-memory option for local development and tests.

Parent initiative: #100

Scope

In Scope

• Implement datastore interface using go-memdb.
• Wire backend registration/selection into runtime datastore config.
• Provide schema/index definitions for required API entities.
• Add unit/integration tests validating behavior parity with abstraction contract.

Out of Scope

• Persistence across restarts.
• Horizontal scaling guarantees.
• Advanced query optimization beyond required API behavior.

Acceptance Criteria

• go-memdb backend satisfies the datastore interface contract used by gitstore-api.
• Runtime config value (e.g. memdb) selects this backend successfully.
• CRUD operations required by API pass contract tests against go-memdb.
• Error behavior is documented and consistent with other backends where applicable.

Tracking

• Milestone: TBD
• Parent: [Initiative] API Datastore Abstraction with Config-Selected Runtime Backend #100
• Labels: initiative, enhancement, area/infra, priority/p2