One discovery subscription over the shared connection for the user's
current zone; matches each incoming offer against saved searches and
fires a text-free OS alert on the new 'alerts' channel for fresh matches.
Skips own/blocked/hidden/non-active offers and dedups via seen keys. The
replay-on-connect doubles as the catch-up scan. Notification payloads are
now prefixed so taps route to chat or the saved search.
