feat: optimize mockup images with WebP and auto-regeneration

Convert mockup source images from JPG to WebP format for 76% size
reduction (20MB → 4.7MB). Variants are now auto-generated on startup
via Oban, keeping the same DRY approach as database images.

Changes:
- Add OptimizeWorker.enqueue_mockup/1 for filesystem images
- Extend VariantCache to check mockup sources on startup
- Update MockupGenerator to save source as optimized WebP
- Update .gitignore to ignore generated variants
- Convert 55 source mockups from JPG to WebP

The mockup pipeline now uses the same code paths as database images:
- Optimizer.to_optimized_webp/1 for source conversion
- Optimizer.process_file/3 for variant generation
- OptimizeWorker for Oban background processing
- VariantCache for startup cache validation

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

lib/simpleshop_theme/printify/mockup_generator.ex

@@ -328,20 +328,35 @@ defmodule SimpleshopTheme.Printify.MockupGenerator do
   end
 
   @doc """
-  Download mockup images to the output directory.
+  Download mockup images, save as WebP source, and generate variants.
+  Sources are saved for regeneration on startup via VariantCache.
   """
   def download_mockups(product_slug, mockup_urls) do
+    alias SimpleshopTheme.Images.Optimizer
+
     File.mkdir_p!(@output_dir)
 
     mockup_urls
     |> Enum.with_index(1)
     |> Enum.map(fn {url, index} ->
-      output_path = Path.join(@output_dir, "#{product_slug}-#{index}.jpg")
-      IO.puts("  Downloading mockup #{index} to #{output_path}...")
+      basename = "#{product_slug}-#{index}"
+      source_path = Path.join(@output_dir, "#{basename}.webp")
+      IO.puts("  Processing mockup #{index}...")
 
-      case Client.download_file(url, output_path) do
-        {:ok, path} -> {:ok, path}
-        {:error, reason} -> {:error, {url, reason}}
+      temp_path = Path.join(System.tmp_dir!(), "#{basename}-temp.jpg")
+
+      with {:ok, _} <- Client.download_file(url, temp_path),
+           {:ok, image_data} <- File.read(temp_path),
+           {:ok, webp_data, source_width, _} <- Optimizer.to_optimized_webp(image_data),
+           :ok <- File.write(source_path, webp_data),
+           {:ok, _} <- Optimizer.process_file(webp_data, basename, @output_dir) do
+        File.rm(temp_path)
+        IO.puts("    Saved source + variants for #{basename} (#{source_width}px)")
+        {:ok, basename, source_width}
+      else
+        {:error, reason} ->
+          File.rm(temp_path)
+          {:error, {url, reason}}
       end
     end)
   end