UwUMail Server 0.30.1

First published release of 0.30: the tag build of 0.30.0 stopped before it published anything,
because its amd64 binary still did not start on CPUs without AVX (a build cache from the
ubuntu-26.04 runner). 0.30.1 is 0.30.0 built afresh; the notes for 0.30.0 below apply.