Critical Notes 1.0.0-rc.2

Second release candidate. It fixes one bug found while testing rc.1.

• Backups: two backups could, in a rare race, both think they held the backup lock after a crashed backup left a stale
  lock behind. Taking over a stale lock is now serialised.

What to test is the same as for rc.1: the image on linux/arm64, an update on a volume that holds data, and phone-width
screens for players on a real phone. A release candidate publishes only its own tag; it does not move 1.0 or latest.