TWO_FACTOR_ENCRYPTION_KEY has no default, so instances upgrading to 3.3
without setting it hit an unhandled throw from encryptSecret: the setup
dialog only reported "Something went wrong", and where 2FA was required
the blocking dialog made the whole app unusable. The status endpoint now
reports 2FA as not required while the key is missing or invalid, and
setup initiation returns 503 with a two-factor-error-not-configured code
that the setup dialog renders as a readable message. Also documents the
variable in the README and the example compose file.
Closes#4443
Francis Cao