- 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.
peter
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