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/tests/menu/resolve_week_start_test.go

@@ -0,0 +1,61 @@
+package menu_test
+
+import (
+	"testing"
+
+	"github.com/food-ai/backend/internal/domain/menu"
+)
+
+func TestResolveWeekStart_ValidFormat(t *testing.T) {
+	// ISO week 1 of 2026 starts on Monday 2025-12-29
+	result, resolveError := menu.ResolveWeekStart("2026-W01")
+	if resolveError != nil {
+		t.Fatalf("unexpected error: %v", resolveError)
+	}
+	if result != "2025-12-29" {
+		t.Errorf("ResolveWeekStart(2026-W01) = %q, want %q", result, "2025-12-29")
+	}
+}
+
+func TestResolveWeekStart_Week10(t *testing.T) {
+	// Week 10 of 2026 starts on Monday 2026-03-02
+	result, resolveError := menu.ResolveWeekStart("2026-W10")
+	if resolveError != nil {
+		t.Fatalf("unexpected error: %v", resolveError)
+	}
+	if result != "2026-03-02" {
+		t.Errorf("ResolveWeekStart(2026-W10) = %q, want %q", result, "2026-03-02")
+	}
+}
+
+func TestResolveWeekStart_InvalidFormat(t *testing.T) {
+	_, resolveError := menu.ResolveWeekStart("2026-01-05")
+	if resolveError == nil {
+		t.Fatal("expected error for date format input, got nil")
+	}
+}
+
+func TestResolveWeekStart_InvalidWeekNum(t *testing.T) {
+	_, resolveError := menu.ResolveWeekStart("2026-W99")
+	if resolveError == nil {
+		t.Fatal("expected error for week 99, got nil")
+	}
+}
+
+func TestResolveWeekStart_WeekZero(t *testing.T) {
+	_, resolveError := menu.ResolveWeekStart("2026-W00")
+	if resolveError == nil {
+		t.Fatal("expected error for week 0, got nil")
+	}
+}
+
+func TestResolveWeekStart_EmptyReturnsCurrentMonday(t *testing.T) {
+	// Empty string should return current week's Monday without error.
+	result, resolveError := menu.ResolveWeekStart("")
+	if resolveError != nil {
+		t.Fatalf("unexpected error: %v", resolveError)
+	}
+	if len(result) != 10 {
+		t.Errorf("expected date in YYYY-MM-DD format, got %q", result)
+	}
+}