config: use /var/lib/dnswatcher as default data directory

Change the default DATA_DIR from the relative path './data' to the
absolute path '/var/lib/dnswatcher', following the FHS convention for
application state data.

The default is derived from the application name passed to setupViper,
so it will always match /var/lib/<appname>.

Updated README environment variable table and .env example to reflect
the new default. Tests updated accordingly.

internal/config/config.go

@@ -94,7 +94,7 @@ func setupViper(name string) {
 
 	viper.SetDefault("PORT", defaultPort)
 	viper.SetDefault("DEBUG", false)
-	viper.SetDefault("DATA_DIR", "./data")
+	viper.SetDefault("DATA_DIR", "/var/lib/"+name)
 	viper.SetDefault("TARGETS", "")
 	viper.SetDefault("SLACK_WEBHOOK", "")
 	viper.SetDefault("MATTERMOST_WEBHOOK", "")