Check a post, or save it as a draft

With ?dryRun=1: run every check and save nothing; data is { dryRun: true, precheck }. This costs no quota and is the call to repeat while revising.

Without it: save a draft and return { post, precheck }. A draft is saved even when its check still has errors (so it can be fixed in place with PATCH); what cannot be saved is a document with no frontmatter, no title, no description, an empty body or an invalid slug, which answers 422.

Figures in the body must be images uploaded to this post, so a figure always fails the check until the draft exists and the image is uploaded — create first, upload, then PATCH.