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/diary/mocks/repository.go

@@ -0,0 +1,26 @@
+package mocks
+
+import (
+	"context"
+
+	"github.com/food-ai/backend/internal/domain/diary"
+)
+
+// MockDiaryRepository is a test double implementing diary.DiaryRepository.
+type MockDiaryRepository struct {
+	ListByDateFn func(ctx context.Context, userID, date string) ([]*diary.Entry, error)
+	CreateFn     func(ctx context.Context, userID string, req diary.CreateRequest) (*diary.Entry, error)
+	DeleteFn     func(ctx context.Context, id, userID string) error
+}
+
+func (m *MockDiaryRepository) ListByDate(ctx context.Context, userID, date string) ([]*diary.Entry, error) {
+	return m.ListByDateFn(ctx, userID, date)
+}
+
+func (m *MockDiaryRepository) Create(ctx context.Context, userID string, req diary.CreateRequest) (*diary.Entry, error) {
+	return m.CreateFn(ctx, userID, req)
+}
+
+func (m *MockDiaryRepository) Delete(ctx context.Context, id, userID string) error {
+	return m.DeleteFn(ctx, id, userID)
+}