Add setting for default Open Graph image fallback

[ellm]

Description

When Open Graph metadata is generated and a post/page does not have a featured image (or other OG image source), no og:image is output. This can result in inconsistent or missing link previews across social platforms.

It would be helpful to have a configurable fallback image that is used whenever an explicit image cannot be determined.

Use Case

Platforms such as Facebook, X, LinkedIn, and Slack rely on og:image for link previews. A fallback ensures every shared URL renders predictably.