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>

.gitignore

@@ -45,11 +45,20 @@ simpleshop_theme-*.tar
 /priv/static/*.gz
 # Digested fonts have 32-char hash before extension
 /priv/static/fonts/*-????????????????????????????????.woff2
-/priv/static/mockups/*-*.jpg
 /priv/static/images/*-*.svg
 /priv/static/images/*-*.svg.gz
 /priv/static/images/*.gz
 
+# Generated mockup variants (auto-generated on startup via Oban)
+# Source .webp files are tracked, variants are regenerated
+/priv/static/mockups/*-400.*
+/priv/static/mockups/*-800.*
+/priv/static/mockups/*-1200.*
+/priv/static/mockups/*-thumb.jpg
+
+# Generated image variants cache (regenerated from source images)
+/priv/static/image_cache/
+
 # In case you use Node.js/npm, you want to ignore these.
 npm-debug.log
 /assets/node_modules/