v0.2.0

Added

• A person's history is kept as sealed records, so a new device reads
  it. POST /v1/messaging/conversations/{id}/backup stores records
  that a person's devices sealed under their backup key, one for each
  message, and GET on the same path lists a person's own records of the
  conversation, oldest after a cursor or newest before it. A record the
  person's devices store twice is kept once. The server sees ciphertext,
  sizes and ids alone; one person's records hold at most 256 MiB, and the
  retention of the log never deletes them. Both calls are authorized as
  backup.recovery. Needs the database migration 000005.