consolidate image serving and clean up pipeline

Move all image URL logic into ProductImage.url/2 and thumbnail_url/1,
remove dead on-demand generation code from Optimizer, strip controller
routes down to SVG recolor only, fix mockup startup check to verify all
variant formats, and isolate test image cache directory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/test.exs

@@ -40,3 +40,6 @@ config :phoenix_live_view,
 
 # Use inline testing mode for Oban
 config :simpleshop_theme, Oban, testing: :inline
+
+# Isolate image cache so test cleanup doesn't wipe the dev cache
+config :simpleshop_theme, :image_cache_dir, Path.expand("../tmp/test_image_cache", __DIR__)