backups: add more info, keep 1 of each version opt

backup's filename format has changed, and includes the commit too now. a
Backup struct for going to/from the filename has been added, and the
option "keep 1 backup from each version" has been added, leaving the
most recent backup from each version always. All pre-this-commit backups
are considered the same "old" version.

config.go

@@ -141,6 +141,7 @@ func (app *appContext) loadConfig() error {
 	app.MustSetValue("backups", "every_n_minutes", "1440")
 	app.MustSetValue("backups", "path", filepath.Join(app.dataPath, "backups"))
 	app.MustSetValue("backups", "keep_n_backups", "20")
+	app.MustSetValue("backups", "keep_previous_version_backup", "true")
 
 	app.config.Section("jellyfin").Key("version").SetValue(version)
 	app.config.Section("jellyfin").Key("device").SetValue("jfa-go")