Three bugs that combined to corrupt DM identities during initial sync:
1. Sync endpoint omitted federatedId for group DMs, causing the receiver
to treat them as 1-on-1 DMs and compute wrong pair hashes — creating
phantom channels that collide with real ones.
2. backfillHomeUserId unconditionally overwrote existing homeUserIds,
so a single wrong match would permanently corrupt a user's identity
and cascade to all subsequent lookups.
3. Migration duplicate-stub Criterion 1 ("shared 1-on-1 DM membership")
incorrectly merged different users from the same domain who were
simply having a conversation, destroying one user's identity.
Jannis Braun
a496dc01bd