Renames dm_channels.canonical_pair_id → federated_id and adds
owner_home_user_id, owner_home_instance, deleted_at columns to support
federated group DMs. Migration handles upgrade from existing installs
(full table rebuild to rename), partial states (ALTER TABLE), and fresh
installs (correct DDL from the start). Backfills federated_id for
existing 1-on-1 DM channels and owner federation identity for group DMs.
Jannis Braun
d9dc33bd36