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/scheduler.py

@@ -272,6 +272,13 @@ def _is_running(job_id: str) -> bool:
         return bool(_activity.get(job_id, {}).get("running"))
 
 
+def running_job_ids() -> set[str]:
+    """Ids of jobs executing right now (any trigger). Lets non-admin surfaces — e.g. the
+    header's sync indicator — reflect real activity instead of just pending-work counts."""
+    with _activity_lock:
+        return {jid for jid, a in _activity.items() if a.get("running")}
+
+
 def trigger_job(job_id: str, actor_id: int | None = None) -> str:
     """Run a job immediately in a background thread, independent of its interval schedule.
     The wrapper handles its own DB session, activity tracking and pause-skip, so the live