Adds column migrations (dm_channels.canonical_pair_id, dm_messages.source_*,
attachments.source_url, instance_settings.federation_relay_*), CREATE TABLE
for federation_peers/outbox/file_queue/mutation_log, partial/filtered indexes,
and an idempotent backfill that seeds the mutation log from existing DM messages.
Also makes migrateDmMessagesReplyToFk and migrateAttachmentsDmMessageFk
column-aware so table recreation preserves any columns added by earlier
migrations (prevents dropping federation columns on upgrade).
Jannis Braun
4726eeb097