- list_channels no longer computes the Normal/Shorts/Live aggregation the
frontend stopped reading when R14 dropped the N/S/L table column; the dead
ManagedChannel.count_* fields are gone too.
- Immediate "get full history" backfill now coordinates with the paced
scheduler via a small in-flight set: capped at 2 concurrent + per-channel
dedup, and run_deep_backfill skips a channel an immediate backfill owns —
killing the cursor race and bounding the quota-drain burst, without losing
the speed win.
- Add test_channel_backfill.py: the channelDetail N/S/L counts + the guard's
no-op / cap / scheduler-skip behaviour.
- Inline the syncedChannels useMemo (its dep is a fresh array each render, so
it never cached); fix the stale "~2-min ticks" comment (it's 10 minutes).
