-- Phase 5C: per-conversation "notify only on @mentions" toggle.
-- Lives alongside the existing muted_until column on conversation_members.
-- When true: the renderer's incoming-message notification gate suppresses
-- the alert unless the message contains an @-mention of the local user.
-- Mentions always fire regardless (override-by-design via the independent
-- useMentionNotifications subscription on message_mentions).

alter table public.conversation_members
  add column if not exists mentions_only boolean not null default false;