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)

models/image_variant.go

@@ -0,0 +1,27 @@
+package models
+
+import "time"
+
+// ImageVariant is one size of a featured image on
+// golem15_journal_image_variants (P33-43): the thumbnail of the original at
+// Width by Height (Key, in the original's format) and its WebP copy
+// (WebPKey), both blob keys of the uploads bucket. SourceWidth and
+// SourceHeight are the original's size. One row per file and width, so the
+// API lists a post's variants with one query instead of storage lookups.
+type ImageVariant struct {
+	ID           uint      `gorm:"column:id;primaryKey"`
+	FileID       uint      `gorm:"column:file_id"`
+	Width        int       `gorm:"column:width"`
+	Height       int       `gorm:"column:height"`
+	Key          string    `gorm:"column:key"`
+	WebPKey      string    `gorm:"column:webp_key"`
+	SourceWidth  int       `gorm:"column:source_width"`
+	SourceHeight int       `gorm:"column:source_height"`
+	CreatedAt    time.Time `gorm:"column:created_at"`
+}
+
+func (ImageVariant) TableName() string { return "golem15_journal_image_variants" }
+
+func init() {
+	Register(ImageVariant{})
+}