feat: add image metadata fields for optimization pipeline

- Add source_width, source_height, variants_status fields to images table
- Remove thumbnail_data (now derived to disk cache)
- Add Oban tables via Oban.Migration.up(version: 12)
- Update Image schema changeset to include new fields

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

priv/repo/migrations/20260121220351_add_image_metadata_and_oban.exs

@@ -0,0 +1,15 @@
+defmodule SimpleshopTheme.Repo.Migrations.AddImageMetadataAndOban do
+  use Ecto.Migration
+
+  def change do
+    alter table(:images) do
+      add :source_width, :integer
+      add :source_height, :integer
+      add :variants_status, :string, default: "pending"
+
+      remove :thumbnail_data
+    end
+
+    Oban.Migration.up(version: 12)
+  end
+end