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/recognition/merge_test.go

@@ -0,0 +1,90 @@
+package recognition_test
+
+import (
+	"testing"
+
+	"github.com/food-ai/backend/internal/adapters/ai"
+	"github.com/food-ai/backend/internal/domain/recognition"
+)
+
+func TestMergeAndDeduplicate_SingleBatch(t *testing.T) {
+	batches := [][]ai.RecognizedItem{
+		{
+			{Name: "apple", Quantity: 2, Unit: "pcs", Confidence: 0.9},
+			{Name: "banana", Quantity: 3, Unit: "pcs", Confidence: 0.8},
+		},
+	}
+
+	result := recognition.MergeAndDeduplicate(batches)
+
+	if len(result) != 2 {
+		t.Fatalf("expected 2 items, got %d", len(result))
+	}
+}
+
+func TestMergeAndDeduplicate_Duplicate(t *testing.T) {
+	batches := [][]ai.RecognizedItem{
+		{{Name: "apple", Quantity: 2, Unit: "pcs", Confidence: 0.7}},
+		{{Name: "apple", Quantity: 3, Unit: "pcs", Confidence: 0.8}},
+	}
+
+	result := recognition.MergeAndDeduplicate(batches)
+
+	if len(result) != 1 {
+		t.Fatalf("expected 1 item after dedup, got %d", len(result))
+	}
+	if result[0].Quantity != 5 {
+		t.Errorf("expected quantity 5 (2+3), got %v", result[0].Quantity)
+	}
+	if result[0].Confidence != 0.8 {
+		t.Errorf("expected confidence 0.8 (higher), got %v", result[0].Confidence)
+	}
+}
+
+func TestMergeAndDeduplicate_CaseInsensitive(t *testing.T) {
+	batches := [][]ai.RecognizedItem{
+		{{Name: "Apple", Quantity: 1, Unit: "pcs", Confidence: 0.9}},
+		{{Name: "apple", Quantity: 2, Unit: "pcs", Confidence: 0.7}},
+	}
+
+	result := recognition.MergeAndDeduplicate(batches)
+
+	if len(result) != 1 {
+		t.Fatalf("expected 1 item (case-insensitive dedup), got %d", len(result))
+	}
+	if result[0].Quantity != 3 {
+		t.Errorf("expected quantity 3, got %v", result[0].Quantity)
+	}
+}
+
+func TestMergeAndDeduplicate_KeepsHigherConfidence(t *testing.T) {
+	batches := [][]ai.RecognizedItem{
+		{{Name: "milk", Quantity: 1, Unit: "L", Confidence: 0.5}},
+		{{Name: "milk", Quantity: 1, Unit: "L", Confidence: 0.95}},
+	}
+
+	result := recognition.MergeAndDeduplicate(batches)
+
+	if len(result) != 1 {
+		t.Fatalf("expected 1 item, got %d", len(result))
+	}
+	if result[0].Confidence != 0.95 {
+		t.Errorf("expected confidence 0.95, got %v", result[0].Confidence)
+	}
+}
+
+func TestMergeAndDeduplicate_Empty(t *testing.T) {
+	result := recognition.MergeAndDeduplicate([][]ai.RecognizedItem{})
+
+	if len(result) != 0 {
+		t.Errorf("expected empty result, got %d items", len(result))
+	}
+}
+
+func TestMergeAndDeduplicate_NilBatches(t *testing.T) {
+	batches := [][]ai.RecognizedItem{nil, nil}
+	result := recognition.MergeAndDeduplicate(batches)
+	if len(result) != 0 {
+		t.Errorf("expected empty result for nil batches, got %d items", len(result))
+	}
+}