feat(backend): DMs, reactions, presence, per-user permissions, chat upgrades
- Private 1:1 conversations: find-or-create, message list (marks read),
send with notifications, edit/delete, message search, unread counts
- Emoji reactions with toggle endpoint + realtime reaction:updated events
- Global online presence (presence:online) + chat/dm typing indicators
and dm:join rooms; chat messages gain attachments, edit timestamps,
search endpoint and edit route
- Per-user permission overrides (allow/deny/inherit) that beat group
rights; workspace admin endpoints to view and set them
x3
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