docs(tests): clarify why fake base64 is acceptable in allowed-image-type test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/test/java/com/recipeapp/recipe/RecipeServiceTest.java

@@ -589,6 +589,8 @@ class RecipeServiceTest {
         when(householdRepository.findById(HOUSEHOLD_ID)).thenReturn(Optional.of(household));
         when(recipeRepository.save(any(Recipe.class))).thenAnswer(i -> i.getArgument(0));
 
+        // "abc" is not valid base64 for a real image; ImageCompressor will return null for the
+        // preview, but validateHeroImageUrl() should pass for a well-formed data URI prefix.
         var request = new RecipeCreateRequest(
                 "Test", null, null, "easy", "data:image/jpeg;base64,abc",
                 List.of(), List.of(), List.of());