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/recognition/handler.go

@@ -134,7 +134,7 @@ func (h *Handler) RecognizeProducts(w http.ResponseWriter, r *http.Request) {
 	}
 	wg.Wait()
 
-	merged := mergeAndDeduplicate(allItems)
+	merged := MergeAndDeduplicate(allItems)
 	enriched := h.enrichItems(r.Context(), merged)
 	writeJSON(w, http.StatusOK, map[string]any{"items": enriched})
 }
@@ -258,9 +258,9 @@ func (h *Handler) saveClassification(ctx context.Context, c *ai.IngredientClassi
 	return saved
 }
 
-// mergeAndDeduplicate combines results from multiple images.
+// MergeAndDeduplicate combines results from multiple images.
 // Items sharing the same name (case-insensitive) have their quantities summed.
-func mergeAndDeduplicate(batches [][]ai.RecognizedItem) []ai.RecognizedItem {
+func MergeAndDeduplicate(batches [][]ai.RecognizedItem) []ai.RecognizedItem {
 	seen := make(map[string]*ai.RecognizedItem)
 	var order []string