[Initiative] Datastore Backend: ScyllaDB with Migrations

[juliuskrah]

Summary

Implement a ScyllaDB datastore backend for the API datastore abstraction, including migration support for schema lifecycle management.

Parent initiative: #100

Scope

In Scope

• Implement datastore interface using ScyllaDB/CQL.
• Add backend registration/selection via runtime datastore config.
• Implement migration support (up/down or versioned forward migrations) for required datastore schema.
• Provide migration execution strategy at startup (safe/no-op when up to date).
• Add integration tests (including migration path) for CRUD contract compliance.

Out of Scope

• Multi-region replication strategy.
• ScyllaDB operational tuning for production workloads.
• Data migration from legacy stores beyond schema migration support.

Acceptance Criteria

• ScyllaDB backend satisfies datastore interface contract used by gitstore-api.
• Runtime config value (e.g. scylladb) selects this backend successfully.
• Migration tooling can initialize and evolve schema safely.
• Startup path handles pending/applied migrations with clear logging and failure semantics.
• Integration tests validate both fresh setup and upgrade migration scenarios.

Tracking

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