fix(header): show sync activity only while a job is actually running

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.

backend/app/routes/sync.py

@@ -13,6 +13,7 @@ from app.sync.runner import (
     run_rss_poll,
 )
 from app.sync.subscriptions import import_subscriptions
+from app.scheduler import running_job_ids
 
 router = APIRouter(prefix="/api/sync", tags=["sync"])
 
@@ -170,6 +171,9 @@ def my_status(
         "quota_used_today": quota.units_used_today(db),
         "quota_remaining_today": quota.remaining_today(db),
         "paused": state.is_sync_paused(db),
+        # True only while a job that advances this view (channel sync) is actually running,
+        # so the header shows live activity rather than a perpetual spinner over pending work.
+        "sync_active": bool({"backfill", "rss_poll"} & running_job_ids()),
     }