deep_requested is per-subscription, but deep backfill is channel-wide and shared:
once any subscriber requests full history, the whole back-catalog arrives for
everyone. The chip only looked at the current user's flag, so a second subscriber
saw a misleading 'get full history' on a channel already queued by someone else.
Add a channel-level deep_in_queue to /api/channels and show an informational
'full history queued' badge (vs the owner's cancelable button) in that case.
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