ci: build backend images with ko and embedded migrations

[prayujt]

Summary

Build AMD64 and ARM64 Go images with ko and persistent compiler caching, preserving the registry tags and Nimbus deployment behavior. Embed the existing SQL migrations so container startup no longer depends on files copied by a Dockerfile.

Test plan

• Built both image architectures; go build ./..., go vet ./..., and formatting checks passed. Started the image against an isolated PostgreSQL container, confirmed migration version 11 and a successful /healthz response.
• Actionlint passed with pre-existing deprecated-action warnings excluded; git diff --check passed.

CI built and published both architectures and completed the Nimbus preview deployment successfully.