Removed mobiledoc rendering and HTML-to-mobiledoc conversion

[kevinansfield]

ref https://linear.app/ghost/issue/GVA-816

Ghost moved to Lexical years ago, but ghost/core still carried two legacy mobiledoc paths that only ran for rare edge cases:

• HTML → mobiledoc — ?source=html (and data import) converted incoming HTML to mobiledoc
• mobiledoc → HTML — content stored only as mobiledoc had its HTML (re)generated by rendering the mobiledoc directly

Both relied on @tryghost/html-to-mobiledoc and @tryghost/kg-mobiledoc-html-renderer, which keep mobiledoc-kit in the dependency tree.

This drops both paths:

• HTML input now always converts to Lexical.
• Any content still stored as mobiledoc is converted to Lexical (via @tryghost/kg-converters) and then rendered, leaving a single rendering path. Posting mobiledoc to the API or importing a legacy mobiledoc export is accepted and auto-converted to Lexical on save (importing runs the cleaned mobiledoc through the model's normal conversion path).
• Removes @tryghost/kg-mobiledoc-html-renderer, @tryghost/html-to-mobiledoc and @tryghost/kg-default-atoms, removing mobiledoc-kit from ghost/core's dependency tree.

Mobiledoc is still stored for legacy posts — the columns, mobiledoc_revisions table, snippet mobiledoc and URL transforms are untouched, so existing mobiledoc content keeps working. Only the conversion and rendering paths are removed.

Behavioural consequences worth calling out:

• Because mobiledoc input is converted to Lexical on save, mobiledoc revisions are no longer created for new content (Lexical/post revisions are created instead).
• The mobiledoc-only populateImageSizes step (which filled in image dimensions on force_rerender) no longer applies; Lexical relies on editor-stored dimensions.

Also fixes a latent connection-pool deadlock in the post model's revision path that surfaced once more content became Lexical (getOwnerUser was called without the surrounding transaction).

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