feat(recipes): add image upload, fix save 500, seed HelloFresh data

- Store hero image as base64 data URI in text column (V023 migration)
- Add file upload UI to RecipeForm with FileReader preview
- Remove isChildFriendly from RecipeCreateRequest (no form field)
- Fix 500 on save: effort values now lowercase, serves/cookTimeMin changed
  from primitive short to nullable Integer to survive omitted fields
- Fix empty categories panel: removed stale tagType=category filter
- Group category tags by type with German headings in recipe form
- Split SuggestionResponse.SuggestionRecipe (no image) from SlotRecipe
- Seed 11 HelloFresh recipes with ingredients, steps and tags (V101)
- Add frontend e2e scaffold, specs and dev yml

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

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

@@ -165,7 +165,7 @@ class RecipeControllerTest {
     private RecipeCreateRequest sampleCreateRequest() {
         var ingredientId = UUID.randomUUID();
         return new RecipeCreateRequest(
-                "Spaghetti Bolognese", (short) 4, (short) 45, "medium", true, null,
+                "Spaghetti Bolognese", 4, 45, "medium", null,
                 List.of(new RecipeCreateRequest.IngredientEntry(
                         ingredientId, null, new BigDecimal("400"), "g", (short) 1)),
                 List.of(new RecipeCreateRequest.StepEntry((short) 1, "Boil water.")),