Add phase 2: recipes with visibility, photos, search and favourites

- Recipe model with ingredients (amount/range, unit, group, note), steps,
  lowercase tags, author notes, source URL and private/public visibility
- Central RecipeQuerySet.visible_to() used by every read path; edit only
  by owner, delete by owner or admin (visible recipes only)
- Text-based editing: ingredient/step parser with roundtrip and scaling
- Photo upload (JPEG/PNG/WebP/HEIC), re-encoded to WebP without EXIF,
  thumbnails, cover image, served only through permission-checked view
- Search across title, description, ingredients, tags; ingredient and tag
  filters; "mine" and favourites views
- Admin can delete all recipes of a deactivated user
- Acceptance tests: private recipes unreachable for others and admins
  in list, search, tags, favourites, detail, edit, delete and image URLs

Co-Authored-By: Claude Opus 5.5 <[email protected]>

static/js/app.js

@@ -12,3 +12,9 @@ document.addEventListener("click", async (event) => {
     button.textContent = "Bitte manuell kopieren";
   }
 });
+
+// Serving calculator: recalculate as soon as the number changes.
+document.addEventListener("change", (event) => {
+  const input = event.target.closest(".servings input[name=portionen]");
+  if (input && input.value) input.form.requestSubmit();
+});