Replace the hard-delete in the leave handler with a soft-delete (sets
deleted_at timestamp) when the last member leaves a group DM. A new
janitor sweep in the federation worker runs hourly and purges channels
whose grace period has expired, cascading through reactions, embeds,
attachments, messages, members, outbox/mutation-log/file-queue entries,
and finally the channel itself.
All client-facing dm_channels queries now filter on deleted_at IS NULL
to hide soft-deleted channels from the REST API and WebSocket ready
payload.
Jannis Braun
4efa35f311