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>

lib/simpleshop_theme_web/live/admin/theme/index.ex

@@ -472,7 +472,7 @@ defmodule SimpleshopThemeWeb.Admin.Theme.Index do
 
     (Map.get(product, :images) || [])
     |> Enum.sort_by(& &1.position)
-    |> Enum.map(fn img -> ProductImage.direct_url(img, 1200) end)
+    |> Enum.map(fn img -> ProductImage.url(img, 1200) end)
     |> Enum.reject(&is_nil/1)
     |> case do
       [] -> []

lib/simpleshop_theme_web/live/admin/theme/index.html.heex

@@ -159,7 +159,7 @@
                 <%= if @logo_image do %>
                   <div class="relative w-16 h-10 bg-base-100 border border-base-300 rounded-lg flex items-center justify-center overflow-hidden">
                     <img
-                      src={"/images/#{@logo_image.id}"}
+                      src={"/image_cache/#{@logo_image.id}.webp"}
                       alt="Current logo"
                       class="max-w-full max-h-full object-contain"
                     />
@@ -299,7 +299,7 @@
             <%= if @header_image do %>
               <div class="relative w-full h-[60px] bg-base-100 border border-base-300 rounded-lg mt-2 overflow-hidden">
                 <img
-                  src={"/images/#{@header_image.id}"}
+                  src={"/image_cache/#{@header_image.id}.webp"}
                   alt="Current header background"
                   class="w-full h-full object-cover"
                 />