The header's sync indicator spun "fetching history" whenever any channel
still lacked full history — i.e. perpetually, even when the scheduler was
idle between its periodic runs. It now spins only while a channel-sync job
(backfill or RSS poll) is actually running; otherwise pending deep-history
work shows as the calm, static "N without full history" link, and recent
work queued for the next run shows a static "N queued". This makes the
header coherent with the Scheduler dashboard's live state.
Backend exposes running_job_ids() from the scheduler activity and a derived
sync_active flag on /api/sync/my-status.
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