test: expand test coverage across diary, product, savedrecipe, ingredient, menu, recognition

- Fix locale_test: add TestMain to pre-populate Supported map so zh/es tests pass
- Export pure functions for testability: ResolveWeekStart, MapCuisineSlug (menu + savedrecipe), MergeAndDeduplicate
- Introduce repository interfaces (DiaryRepository, ProductRepository, SavedRecipeRepository, IngredientSearcher) in each handler; NewHandler now accepts interfaces — concrete *Repository still satisfies them
- Add mock files: diary/mocks, product/mocks, savedrecipe/mocks
- Add handler unit tests (no DB) for diary (8), product (8), savedrecipe (8), ingredient (5)
- Add pure-function unit tests: menu/ResolveWeekStart (6), savedrecipe/MapCuisineSlug (5), recognition/MergeAndDeduplicate (6)
- Add repository integration tests (//go:build integration): diary (4), product (6)
- Extend recipe integration tests: GetByID_Found, GetByID_WithTranslation

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

backend/internal/domain/savedrecipe/repository.go

@@ -34,7 +34,7 @@ func (r *Repository) Save(ctx context.Context, userID string, req SaveRequest) (
 		cr := dish.CreateRequest{
 			Name:        req.Title,
 			Description: req.Description,
-			CuisineSlug: mapCuisineSlug(req.Cuisine),
+			CuisineSlug: MapCuisineSlug(req.Cuisine),
 			ImageURL:    req.ImageURL,
 			Source:      req.Source,
 			Difficulty:  req.Difficulty,
@@ -373,9 +373,9 @@ func scanUSR(s rowScanner) (*UserSavedRecipe, error) {
 	return &r, err
 }
 
-// mapCuisineSlug maps a free-form cuisine string (from Gemini) to a known slug.
+// MapCuisineSlug maps a free-form cuisine string (from Gemini) to a known slug.
 // Falls back to "other".
-func mapCuisineSlug(cuisine string) string {
+func MapCuisineSlug(cuisine string) string {
 	known := map[string]string{
 		"russian":       "russian",
 		"italian":       "italian",