Upgrades keep settings left in the program folder

Settings put in the program folder's appsettings.json (instead of %ProgramData%\MediaRiffle\appsettings.json or /etc/mediariffle/appsettings.json) were lost when an upgrade replaced the program folder. The Windows installer, the .deb (new preinst) and install.sh now save the old file, and the new version runs MediaRiffle.Server --carry-settings : settings that aren't defaults (built-in MediaRiffle defaults, or the new program-folder file for the rest) go into the settings file where it doesn't set them or sets only the default; where it set something itself, it wins. It's backed up first.

The program folder's appsettings.json now holds only logging (the defaults are built in), and the log warns at start about settings left there. Docs, changelog, tests.

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com