fix: createRecipe checks name is unique

Uses JPA Repository.exists(Example<T>); Changed no-arg constuctor of
Recipe to public.

commons/src/main/java/commons/Recipe.java

@@ -83,7 +83,7 @@ public class Recipe {
     private List<String> preparationSteps = new ArrayList<>();
 
     @SuppressWarnings("unused")
-    private Recipe() {
+    public Recipe() {
         // for object mapper
     }