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)

console/image_variants.go

@@ -0,0 +1,67 @@
+package console
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"git.golem15.com/golem15/sm-journal-plugin/classes"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/bonfire"
+	"gocloud.dev/blob"
+)
+
+// ImageVariants is journal:image-variants: it writes the thumbnails and
+// WebP copies of the posts' featured images at golem15.journal.image_widths
+// in this process (the backfill of the golem15.journal.image_variants job).
+// Without --all it takes the images that lack a variant of some width.
+func ImageVariants(app *backpack.App) bonfire.Command {
+	return bonfire.Command{
+		Name:        "journal:image-variants",
+		Description: "Write the WebP and resized variants of featured images",
+		Long: `Writes, for every post's featured image, a thumbnail at each width of
+golem15.journal.image_widths (480, 800 and 1200 by default) that is not
+wider than the original, and a WebP copy of each next to it, and records
+them in golem15_journal_image_variants. The Journal API lists them as
+featured_images[].variants and the journal layer serves them through
+<picture> with the original format as fallback.
+
+New uploads get their variants from the golem15.journal.image_variants job,
+queued after the upload commits. This command is the backfill: without
+--all it takes the images that lack a variant of some width (for example
+after adding a width), with --all it checks every featured image. A
+variant that exists is never written again, so the command can run any
+number of times.`,
+		Flags: []bonfire.Flag{
+			{Name: "all", Description: "Check every featured image (default: only those missing a width)", Bare: true},
+		},
+		Examples: []bonfire.Example{
+			{Command: "journal:image-variants", Description: "Write the variants images are missing"},
+			{Command: "journal:image-variants --all", Description: "Check every featured image"},
+		},
+		Run: func(ctx context.Context, in bonfire.Input, out bonfire.Output) error {
+			db := mustDB(app)
+			if db == nil {
+				return errors.New("journal: database is not configured")
+			}
+			bucket, ok := app.Lookup[*blob.Bucket]()
+			if !ok || bucket == nil {
+				return errors.New("journal: the uploads bucket is not configured")
+			}
+			ids, err := classes.FeaturedImageIDs(db.WithContext(ctx), appConfig(app), flagOn(in, "all"))
+			if err != nil {
+				return err
+			}
+			if len(ids) == 0 {
+				out.Info("Every featured image has its variants.")
+				return nil
+			}
+			res, err := classes.GenerateImageVariants(ctx, db, bucket, classes.ImageWidths(appConfig(app)), ids)
+			if err != nil {
+				return err
+			}
+			out.Success(fmt.Sprintf("Checked %d featured images, wrote %d variants, skipped %d.", res.Files, res.Written, res.Skipped))
+			return nil
+		},
+	}
+}