fix(server): use fqdn for og:image meta tag value

[eleith]

opengraph image specifies that the url contains http or https, thus implying a fqdn.

this change uses the external domain from the server config to attempt to make the og:image have both the existing path to the thumbnail along with the desired domain

please note, some og implementations do work with relative paths, so not all og image checkers may still pass, but not all implementations have this fallback and thus will not find the image otherwise

fix #11081