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)

controllers/api/posts.go

@@ -468,6 +468,8 @@ func UploadFeaturedImage(app *backpack.App) http.HandlerFunc {
 		stored.AttachmentID = idStr
 		stored.Field = "featured_images"
 		purgeSiteCache(r, app, post)
+		// The variants are written by a job, never in this request (D-85).
+		classes.DispatchImageVariants(r.Context(), app, []uint{stored.ID})
 		wire.WriteJSON(w, http.StatusCreated, map[string]any{
 			"data": map[string]any{
 				"id":        stored.ID,
@@ -535,6 +537,10 @@ func DeleteFeaturedImage(app *backpack.App) http.HandlerFunc {
 			writeAPIError(w, http.StatusInternalServerError, "Internal server error")
 			return
 		}
+		// The variants' blobs share the thumbnail prefix of BlobKeys.
+		if err := db.WithContext(r.Context()).Where("file_id = ?", file.ID).Delete(&models.ImageVariant{}).Error; err != nil {
+			pluginLog.Warn("journal: delete image variants", "file", file.ID, "error", err)
+		}
 		if bucket, ok := app.Lookup[*blob.Bucket](); ok && bucket != nil {
 			_ = attach.DeleteKeys(r.Context(), bucket, keys)
 		}
@@ -761,13 +767,48 @@ func featuredImages(db *gorm.DB, p *models.Post) []map[string]any {
 	).Order("sort_order ASC, id ASC").Find(&files).Error; err != nil {
 		return []map[string]any{}
 	}
+	variants := imageVariants(db, files)
 	out := make([]map[string]any, 0, len(files))
 	for i := range files {
+		list := variants[files[i].ID]
+		if list == nil {
+			list = []map[string]any{}
+		}
 		out = append(out, map[string]any{
 			"id":        files[i].ID,
 			"url":       files[i].URL(),
 			"file_name": files[i].FileName,
 			"file_size": files[i].FileSize,
+			"variants":  list,
+		})
+	}
+	return out
+}
+
+// imageVariants lists the variants of files in one query, by file and
+// width (D-85): {width, height, url, webp_url} and, on each, the
+// original's source_width and source_height.
+func imageVariants(db *gorm.DB, files []attach.File) map[uint][]map[string]any {
+	out := map[uint][]map[string]any{}
+	if len(files) == 0 {
+		return out
+	}
+	ids := make([]uint, 0, len(files))
+	for _, f := range files {
+		ids = append(ids, f.ID)
+	}
+	var rows []models.ImageVariant
+	if err := db.Where("file_id IN ?", ids).Order("file_id ASC, width ASC").Find(&rows).Error; err != nil {
+		return out
+	}
+	for _, v := range rows {
+		out[v.FileID] = append(out[v.FileID], map[string]any{
+			"width":         v.Width,
+			"height":        v.Height,
+			"url":           attach.PublicURL(v.Key),
+			"webp_url":      attach.PublicURL(v.WebPKey),
+			"source_width":  v.SourceWidth,
+			"source_height": v.SourceHeight,
 		})
 	}
 	return out