See comment in #784 (comment)

Datasette needs to be able to set signed cookies - which means it needs a mechanism for safely handling a signing secret.

Since Datasette is a long-running process the default behaviour here can be to create a random secret on startup. This means that if the server restarts any signed cookies will be invalidated.

If the user wants a persistent secret they'll have to generate it themselves - maybe by setting an environment variable?