- add chat_notification_settings table and migration
- add chat notifications API (get/update muted)
- skip message notifications for muted recipients
- add mute/unmute control in chat info panel
- block send/forward in channels for member role on backend
- expose my_role in chat payload for client-side permissions
- hide message composer for channel members and show read-only notice
- hide members management sections for private/saved chats in info panel
- return enriched chat detail via serialize_chat_for_user
- add user last_seen_at with alembic migration and persist on realtime disconnect
- extend chat serialization with private online/last_seen, group members/online, channel subscribers
- add Redis batch presence lookup helper
- update web chat list/header to display status counters and last-seen labels
- improve delivery receipt handling using last_delivered/last_read boundaries
- include chat info panel and related API/type updates