Auto-reset: server-side reset via external tool

[askmeaboutlo0m]

There's not really a need for a client doing the session reset, it could just as well be done server-side using a (separate, versionable) tool that replays the session history up to the current point and then sending the resulting canvas to the server.

More talk about this at https://docs.drawpile.net/devblog/2024/03/03/resets.html#server-side-reset-solution