A scrape-time collector reports scimesh_tasks/jobs/workers gauges keyed by
status, sourced from cheap GROUP BY queries (StatsRepo), zero-filled across all
known statuses so the dashboard shows flat zeros instead of gaps. A failed
query yields no samples for that scrape rather than crashing it.
Metrics is now built in main so the DB-backed collector can be registered
(NewServer takes *metrics.Metrics; nil self-provisions for tests). Grafana
dashboard gains a Domain state row: tasks/jobs/workers by status and a queue-
depth stat.