Add phase 3: comments and cooking mode

- Comments on visible recipes; author and admin may delete; per-user
  hourly rate limit (COMMENT_RATE_LIMIT_PER_HOUR)
- Cooking mode with large text, tick-off checklists, serving scaling and
  Screen Wake Lock to keep the display on
- Serving calculator auto-submits on change
- Privacy tests cover comment and cooking-mode URLs

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

recipes/models.py

@@ -197,3 +197,26 @@ class Favorite(models.Model):
 
     class Meta:
         constraints = [models.UniqueConstraint(fields=["user", "recipe"], name="favorite_unique")]
+
+
+class CommentQuerySet(models.QuerySet):
+    def deletable_by(self, user):
+        if user is None or not user.is_authenticated:
+            return self.none()
+        visible = self.filter(recipe__in=Recipe.objects.visible_to(user))
+        return visible if user.is_admin else visible.filter(author=user)
+
+
+class Comment(models.Model):
+    recipe = models.ForeignKey(Recipe, on_delete=models.CASCADE, related_name="comments")
+    author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name="comments")
+    text = models.TextField("Kommentar", max_length=2000)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    objects = CommentQuerySet.as_manager()
+
+    class Meta:
+        ordering = ["created_at"]
+
+    def __str__(self):
+        return f"Kommentar von {self.author} zu {self.recipe}"