Configure Go-based containers for auth and gateway

[karadyaur]

Summary

• rebuild sc-auth as a multi-stage Go image that bundles migrations and runs from a distroless base
• update the auth entrypoint to apply migrations before launching the new binary with environment-driven configuration
• convert the API gateway image to a Go service, make the gRPC target configurable, and add both services to docker-compose with the proper ports

Testing

• go test ./... (hangs locally)

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https://chatgpt.com/codex/tasks/task_e_68da7cbd867c8329975ddc8968a051a9