The previous pass only emailed on a mention. Corrected scope: the
room's first unread message triggers one debounced email (same shape
as #66), and every mention additionally emails regardless of that
debounce, since a mention shouldn't get silently absorbed by an
earlier plain message's already-sent notification.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Lets a member subscribe to email when they're @mentioned in a room
while offline, alongside #66's always-on DM email. Debounced the same
way #66 is (one email per unread burst, not one per mention), and
deliberately scoped to regular rooms only -- DMs already have #66's
automatic offline email with no separate toggle needed.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>