chore: prepare Downloader 1.8.1

[Priveetee]

Summary

• prepare Downloader version 1.8.1
• make shutdown wait for workers before closing persistence
• add race-free job snapshots and durable artifact persistence
• improve PostgreSQL, Dragonfly and S3 lifecycle handling
• update the Garage runtime and multi-architecture image verification

Production behavior

Downloader shutdown drains active workers before closing persistence, job snapshots remain consistent under concurrent access, and completed artifacts keep their durable metadata.

Deployment requirements

Publish the Downloader 1.8.1 image from main and pin the production downloader service to its immutable digest. Preserve the PostgreSQL, Dragonfly, Garage and artifact volumes across the rollout.

Runtime verification

• The published image contains linux/amd64 and linux/arm64 and passes its FFmpeg runtime check.
• A small authenticated download reaches completion and its artifact remains available.
• Shutdown and restart do not lose queued jobs or persisted artifact metadata.
• The existing downloader database and object-storage volumes remain attached.

Tests

• go test ./...
• go build ./...
• Docker CI and Coverage checks through CI.
• gofmt and git diff --check

Rollback

Restore the previous Downloader image digest and submodule revision, then recreate only the typetype-downloader service. Keep existing job and artifact volumes intact.