Virtual merges expire after 60s and the next request blocks on every member. One slow member (a large github_rpm remote) makes the re-merge outlast dnf's 30s timeout.
- add `serve_stale` to remotes (model, v2 API, migration 0008, UI, TUI), default false
- serve a flagged member's last good fetch to rpm/deb/alpine merges at once
- refresh it in the background, single-flighted, detached from the request
- block on the member again once its copy is over an hour old, or the flag is off
- prune copies with their merge cache; drop refreshes for evicted entries
Reviewed-on: #167
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
