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/routes/[recipeLang=recipeLang]/edit/[name]/+page.svelte

@@ -11,7 +11,7 @@
     	export let data: PageData;
 	let preamble = data.recipe.preamble
 	let addendum = data.recipe.addendum
-	let image_preview_url="https://bocken.org/static/rezepte/thumb/" + data.recipe.short_name + ".webp?v=" + data.recipe.dateModified;
+	let image_preview_url="https://bocken.org/static/rezepte/thumb/" + (data.recipe.images?.[0]?.mediapath || `${data.recipe.short_name}.webp`);
 	let note = data.recipe.note
 
 	// Translation workflow state