implement content-hash based image cache invalidation

Add content-based hashing to recipe images for proper cache invalidation
while maintaining graceful degradation through dual file storage.

Changes:
- Add imageHash utility with SHA-256 content hashing (8-char)
- Update Recipe model to store hashed filenames in images[0].mediapath
- Modify image upload endpoint to save both hashed and unhashed versions
- Update frontend components to use images[0].mediapath with fallback
- Add migration endpoint to hash existing images (production-only)
- Update image delete/rename endpoints to handle both file versions

Images are now stored as:
  - recipe.a1b2c3d4.webp (hashed, cached forever)
  - recipe.webp (unhashed, graceful degradation fallback)

Database stores hashed filename for cache busting, while unhashed
version remains on disk for backward compatibility and manual uploads.

src/lib/js/recipeJsonLd.ts

@@ -42,7 +42,7 @@ export function generateRecipeJsonLd(data: any) {
     "keywords": data.tags?.join(', '),
     "image": {
       "@type": "ImageObject",
-      "url": `https://bocken.org/static/rezepte/full/${data.short_name}.webp`,
+      "url": `https://bocken.org/static/rezepte/full/${data.images?.[0]?.mediapath || `${data.short_name}.webp`}`,
       "width": 1200,
       "height": 800
     },