Existing 1-on-1 DM channels created before federation had canonical_pair_id = NULL,
causing the relay to create duplicate channels on receipt of a message. On server
startup, migrations now: (1) compute and set canonical_pair_id for all existing
1-on-1 channels using home_user_id (falling back to regular id for local users),
and (2) merge any duplicate channels that share the same canonical_pair_id by
moving all messages, read states, and members into the oldest channel and deleting
the stale duplicate. Both steps are non-fatal and idempotent.
Jannis Braun
4a109f71da