feat: enhance image optimization with on-demand JPEG fallbacks

Improve the image optimization pipeline with better compression and
smarter variant generation:

- Change to_lossless_webp → to_optimized_webp (lossy, quality 90)
- Auto-resize uploads larger than 2000px to save storage
- Skip pre-generating JPEG variants (~50% disk savings)
- Add on-demand JPEG generation for legacy browsers (<5% of users)
- Add /images/:id/variant/:width route for dynamic serving
- Add VariantCache to supervision tree for startup validation
- Add image_cache to static paths for disk-based serving

The pipeline now stores smaller WebP sources and generates AVIF/WebP
variants upfront, with JPEG generated only when legacy browsers request it.

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

test/support/fixtures/image_fixtures.ex

@@ -11,7 +11,7 @@ defmodule SimpleshopTheme.ImageFixtures do
   def sample_jpeg, do: @sample_jpeg
 
   def image_fixture(attrs \\ %{}) do
-    {:ok, webp, w, h} = SimpleshopTheme.Images.Optimizer.to_lossless_webp(@sample_jpeg)
+    {:ok, webp, w, h} = SimpleshopTheme.Images.Optimizer.to_optimized_webp(@sample_jpeg)
 
     defaults = %{
       image_type: "product",