[Initiative] API Datastore Abstraction with Config-Selected Runtime Backend

[juliuskrah]

Summary

Introduce a datastore abstraction in gitstore-api so runtime storage can be selected by configuration, with an initial implementation targeting two backends.

Inspired by SpiceDB datastore patterns: https://github.com/authzed/spicedb/tree/main/internal/datastore

Scope

In Scope

• Define a datastore interface/contract for API persistence paths.
• Add runtime datastore selection via configuration (e.g. env/config key).
• Support exactly two initial datastore implementations.
• Provide boot-time validation and clear errors for unsupported datastore values.
• Document datastore architecture and configuration options in docs/.

Out of Scope

• Additional datastore backends beyond the first two.
• Advanced multi-datastore routing or hybrid read/write splitting.
• Data migration tooling beyond backend-specific migration support included in subtasks.

Acceptance Criteria

• gitstore-api uses a datastore abstraction instead of direct backend coupling in core persistence paths.
• Runtime config selects the datastore implementation without code changes.
• Unsupported datastore config values fail fast with actionable startup errors.
• Integration tests cover datastore selection and basic CRUD parity across supported backends.
• Documentation explains datastore config and extension points for future backends.

Subtasks

• [Initiative] Datastore Backend: go-memdb #101
• [Initiative] Datastore Backend: ScyllaDB with Migrations #102

Tracking

• Milestone: TBD
• Labels: initiative, enhancement, area/infra, priority/p1