Adds a durable mapping so the UI never has to show raw t2_xxx IDs or
MXIDs just because the in-memory cache is cold after a restart.
ChatService seeds from the table on construction and writes through
every resolution path; cached chats whose username is still t2_xxx
get upgraded from the DB on loadCache.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
