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

Parent

#91

What to build

• Your own messages: edit them (shown "(edited)") and delete them (shown "This message was deleted.").
• Moderation: any Teacher and the Superuser can delete any message.
• Deleting removes the text and the files, but keeps the row so replies can say the original was deleted.
• Reply with a quote: the reply shows a short quote of the original (author + first line) above it, and clicking the quote scrolls to the original. A quote of a deleted message says "This message was deleted.".
• Live: edits and deletions appear live in open chats.

Spec stories 16-20.

Acceptance criteria

• PUT /api/chat/messages/{id} (author only), DELETE /api/chat/messages/{id} (author or staff); reply_to_id on send (same chat only)
• Changes since after include edited and deleted messages
• Frontend: message menu (Edit, Delete, Reply), reply preview in the send box, quote with scroll-to
• API and NiceGUI tests (edit, delete own, staff deletes, Student can't delete others', reply to deleted)

Blocked by

• Chat photos and files, including backup #94