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/locale/locale_test.go

@@ -3,11 +3,21 @@ package locale_test
 import (
 	"context"
 	"net/http"
+	"os"
 	"testing"
 
 	"github.com/food-ai/backend/internal/infra/locale"
 )
 
+func TestMain(m *testing.M) {
+	// Populate Supported map with all known languages before running tests.
+	// In production this is done by locale.LoadFromDB at startup.
+	for _, code := range []string{"en", "ru", "es", "de", "fr", "it", "pt", "zh", "ja", "ko", "ar", "hi"} {
+		locale.Supported[code] = true
+	}
+	os.Exit(m.Run())
+}
+
 func TestParse(t *testing.T) {
 	tests := []struct {
 		name       string