feat(P5C.T2-sql): conversation_members.mentions_only column

supabase/migrations/20260516000010_mentions_only.sql

@@ -0,0 +1,9 @@
+-- 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;