Chat lifecycle: archive, 6-month cleanup, restore, delete, deleted Users #98

Parent

#91

What to build

• Archiving: projects.archived_at records when a Project was archived. Its chat becomes read-only, and only Teachers and the Superuser can still read it; Members, Makers and Guests lose access. The chat shows "This chat will be deleted on " (6 months after archiving).
• Cleanup: once a day, the backend's background loop deletes the messages and files of chats of Projects archived more than 6 months ago.
• Restore within 6 months: the chat becomes writable again, with its history. After the cleanup, it starts empty.
• Deleting a Project deletes its chat and files at once.
• Deleting a User is no longer refused because of chat messages; their messages stay, shown as "Deleted user". A deactivated User's messages stay under their name.

Spec stories 36-41.

Acceptance criteria

• Migration: projects.archived_at (backfilled with now() for already Archived Projects)
• Archived chats refuse sending, editing, reacting, typing and Guests (409); Students and Guests get 403
• Cleanup job + test with a moved clock; files removed from the volume
• Delete Project removes chat files; delete User keeps messages as "Deleted user"
• Frontend: read-only chat with the deletion notice
• API tests; NiceGUI test of the read-only archived chat

Blocked by

• Chat: 'is typing…', 'Seen by' and search #97