fix(sabnzbd): surface arr queue and recent failures

[rexdotsh]

Summary

• Treat SABnzbd queue limit=0 as an unbounded request so Radarr/Sonarr can see active AltMount jobs
• Sort SABnzbd history by newest item after combining queue/history rows so recent failures are not hidden behind older completed items
• Preserve completion timestamps when adapting persistent history rows for SABnzbd history responses

Testing

• go test ./internal/api