fix(channels): address R14 deep-review findings

- Make the immediate-backfill guard SYMMETRIC: run_deep_backfill now also
  registers the channel it pages in _backfill_in_progress, so it and
  backfill_channel_now mutually exclude per-channel and never advance the
  same cursor concurrently (whichever started first). The interactive cap
  moves to a separate _immediate_running counter so the scheduler's channels
  don't false-trip it.
- Attribute the immediate backfill's spend: wrap it in
  quota.attribute(user_id, VIDEOS_BACKFILL_FULL) (route passes user.id), so
  it shows under the user's full-backfill action, not the OTHER/system bucket.
- Tests: the update_channel->background-task wiring + the demo guard
  (scheduled for a real user, NOT for demo); distinct N/S/L counts (2 short
  + 1 live) so a short/live swap fails; extracted channelDetailPollMs with a
  pure unit test of the poll-or-stop predicate.
- a11y: role="img" on the fully-synced charm so screen readers announce it.

frontend/src/components/channelColumns.tsx

@@ -52,6 +52,7 @@ function StoredCell({ c }: { c: ManagedChannel }) {
       {isFullySynced(c) && (
         <Tooltip hint={t("channels.row.fullySyncedHint")}>
           <Check
+            role="img"
             className="w-3.5 h-3.5 text-accent shrink-0 cursor-help"
             aria-label={t("channels.row.fullySynced")}
           />