New /app/account/notifications/preferences page with a matrix: rows = event types, columns = delivery channels (in-app, email, web push, Slack, Discord, SMS, WhatsApp). Each cell is a checkbox. Sensible defaults: mentions go to in-app + email, status changes on watched posts in-app only, weekly digest on by default for non-watchers.

Plus three global controls: quiet hours (no non-critical pings between e.g. 22:00-08:00 local), digest cadence (off / daily / weekly), and a master mute. Watch/follow primitive: users can "watch" a post or board to subscribe to its events; otherwise they only get notified about things addressed to them. Schema: notification_preferences (user_id, event_type, channel, enabled), watches (user_id, target_type, target_id).

Out of scope: actually delivering on email/push/Slack/Discord/SMS - those are separate channel cards. This one only stores the prefs and exposes a query helper "would user X want this event on channel Y?".