chore: Prepare Downloader 1.8.0

[Priveetee]

Summary

• prepare Downloader version 1.8.0
• keep bounded SABR download concurrency and multi-architecture image validation

Production behavior

The Downloader API and storage contract remain unchanged. The release keeps bounded workers, exact stream selection and the verified multi-architecture runtime image flow.

Deployment requirements

Publish the Downloader 1.8.0 image from main and pin the production downloader service to its immutable digest. No database migration is required.

Runtime verification

• /api/downloader/health is healthy after rollout.
• A representative download reaches done and serves its artifact.
• The amd64 and arm64 image checks pass in CI.

Tests

• go test ./...
• go build ./...
• Dockerfile and compose validation
• git diff --check

Rollback

Restore the previous Downloader image digest and the previous Downloader submodule revision, then recreate only typetype-downloader.