Chat reactions and @mentions #96

Parent

#91

What to build

• Reactions: 👍 ❤️ 😂 😮 😢 🎉 on any message. Click to add or remove yours; several per person are allowed. Each reaction shows a count, with the names on hover.
• @mentions: typing @ in the send box lists the chat people, plus @team for all Members.
  • A mentioned person sees the message highlighted.
  • The chat gets a red @ marker on the navigation item and on the Chats screen until they have read past it.
  • Staff who only read along can't be mentioned.
  • No emails.
• Live: reactions appear live in open chats.

Spec stories 21-23. Adds Mention to CONTEXT.md.

Acceptance criteria

• Migrations chat_reactions, chat_mentions
• PUT/DELETE /api/chat/messages/{id}/reactions/{emoji} (six allowed); mentions parsed and validated on send and edit; unread responses include mentioned
• Frontend: reaction bar, counts with names, @ autocomplete, highlight, @ marker
• API and NiceGUI tests

Blocked by

• Chat: edit, delete, moderation and reply with a quote #95