Get mumble-server running on iec

[jhenstridge]

Currently mumble-server is failing to start due to permissions on the TLS keys and certificates:

Oct 14 19:45:23 iec systemd[1]: Started mumble-server.service - Mumble server.
Oct 14 19:45:23 iec mumble-server[308844]: <X>2025-10-14 19:45:23.193 SSL: OpenSSL version is 'OpenSSL 3.5.1 1 Jul 2025'
Oct 14 19:45:23 iec mumble-server[308844]: <W>2025-10-14 19:45:23.195 Initializing settings from /etc/mumble/mumble-server.ini (basepath /etc/mumble)
Oct 14 19:45:23 iec mumble-server[308844]: <C>2025-10-14 19:45:23.195 MetaParams: Failed to read /etc/letsencrypt/live/plug.org.au/fullchain.pem
Oct 14 19:45:23 iec mumble-server[308844]: <F>2025-10-14 19:45:23.195 MetaParams: Failed to load SSL settings. See previous errors.
Oct 14 19:45:23 iec systemd[1]: mumble-server.service: Main process exited, code=exited, status=1/FAILURE

The server is started started as the mumble-server user, which explains why it can't access the certificate. We don't want to change the permissions on /etc/letsencrypt, so the fix should probably look something like:

1. Add a certbot hook that copies the new certificate and key to a location the mumble-server user can access.
2. Adjust mumble-server.ini to point to this location.

There's probably some additional configuration needed to create an admin account and channels. Alternatively, we could just copy Mumble's sqlite database from power