fix(channels): address R14 code-review findings

- 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).

frontend/src/components/channelColumns.dom.test.tsx

@@ -19,9 +19,6 @@ const ch = (over: Partial<ManagedChannel>): ManagedChannel => ({
   stored_videos: 0,
   last_video_at: null,
   total_duration_seconds: 0,
-  count_normal: 0,
-  count_short: 0,
-  count_live: 0,
   priority: 0,
   hidden: false,
   deep_requested: false,