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/product/handler.go

@@ -1,6 +1,7 @@
 package product
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"log/slog"
@@ -10,13 +11,22 @@ import (
 	"github.com/go-chi/chi/v5"
 )
 
+// ProductRepository is the data layer interface used by Handler.
+type ProductRepository interface {
+	List(ctx context.Context, userID string) ([]*Product, error)
+	Create(ctx context.Context, userID string, req CreateRequest) (*Product, error)
+	BatchCreate(ctx context.Context, userID string, items []CreateRequest) ([]*Product, error)
+	Update(ctx context.Context, id, userID string, req UpdateRequest) (*Product, error)
+	Delete(ctx context.Context, id, userID string) error
+}
+
 // Handler handles /products HTTP requests.
 type Handler struct {
-	repo *Repository
+	repo ProductRepository
 }
 
 // NewHandler creates a new Handler.
-func NewHandler(repo *Repository) *Handler {
+func NewHandler(repo ProductRepository) *Handler {
 	return &Handler{repo: repo}
 }