Chat Guests, 'Only my chats' and Students with several chats #93

Parent

#91

What to build

• Guests: the Product Owner, any Teacher or the Superuser adds any active User to a Project's chat as a Guest; Members can't. A Guest reads and writes like the team and sees the full history. They can leave the chat themselves, and the same staff can remove them.
• People list: each chat has a people list with the Members, the Product Owner and the Guests (marked "Guest").
• Badge for Teachers: it now counts "my chats", meaning the Product Owner's chats and the chats where they are a Guest.
• Chats screen: an "Only my chats" filter, on by default.
• Students: a Student's Chat item opens their only chat directly, or a list of their chats (own Project + Guest chats) with unread counts. A Student who is only a Guest (no Project) also gets the item.

Spec stories 5-10, 27, 31, 33. Adds Guest to CONTEXT.md.

Acceptance criteria

• Migration chat_guests
• GET/POST /api/projects/{id}/chat/guests, DELETE /api/projects/{id}/chat/guests/{user_id} (staff, or the Guest leaving); Members refused; inactive Users refused
• GET /api/chats?mine=true; Student chat list
• Frontend: people list, add Guest dialog (select of active Users), Leave chat, Remove, "Only my chats" toggle, Students' chat list
• API and NiceGUI tests (add, write as Guest, leave, remove, Member refused)

Blocked by

• Chat basics: one chat per Project, text messages, live updates, unread badge #92