feat: image variants job and variants in the post API

- golem15_journal_image_variants (migration 202610280002): a thumbnail
  per width of golem15.journal.image_widths (480, 800, 1200 by default,
  never wider than the original) and its WebP copy, with the original's
  size
- conga job golem15.journal.image_variants (three attempts), queued after
  a featured image upload and after an admin save of a post whose images
  lack a width; the upload request never encodes
- journal:image-variants [--all] backfills
- featured_images[].variants {width, height, url, webp_url, source_width,
  source_height} on the post answers (additive key)

plugin.go

@@ -123,6 +123,7 @@ func (p *Plugin) Commands() []bonfire.Command {
 		console.ImportPosts(p.app),
 		console.Reindex(p.app),
 		console.FixHeadings(p.app),
+		console.ImageVariants(p.app),
 	}
 }