feat: implement backend localization infrastructure

- Add internal/locale package: Parse(Accept-Language), FromContext/WithLang helpers, 12 supported languages
- Add Language middleware that reads Accept-Language header and stores lang in context
- Register Language middleware globally in server router (after CORS)

Database migrations:
- 009: create recipe_translations, saved_recipe_translations, ingredient_translations tables; migrate existing _ru data
- 010: drop legacy _ru columns (title_ru, description_ru, canonical_name_ru); update FTS index

Models: remove all _ru fields (TitleRu, DescriptionRu, NameRu, UnitRu, CanonicalNameRu)

Repositories:
- recipe: Upsert drops _ru params; GetByID does LEFT JOIN COALESCE on recipe_translations; ListMissingTranslation(lang); UpsertTranslation
- ingredient: same pattern with ingredient_translations; Search now queries translated names/aliases
- savedrecipe: List/GetByID LEFT JOIN COALESCE on saved_recipe_translations; UpsertTranslation

Gemini:
- RecipeRequest/MenuRequest gain Lang field
- buildRecipePrompt rewritten in English with target-language content instruction; image_query always in English
- GenerateMenu propagates Lang to GenerateRecipes

Handlers:
- recommendation/menu: pass locale.FromContext(ctx) as Lang
- recognition: saveClassification stores Russian translation via UpsertTranslation instead of _ru column

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/ingredient/repository_integration_test.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"testing"
 
+	"github.com/food-ai/backend/internal/locale"
 	"github.com/food-ai/backend/internal/testutil"
 )
 
@@ -65,7 +66,6 @@ func TestIngredientRepository_Upsert_ConflictUpdates(t *testing.T) {
 		t.Fatalf("first upsert: %v", err)
 	}
 
-	// Update with same spoonacular_id
 	cal := 89.0
 	second := &IngredientMapping{
 		CanonicalName:   "banana_updated",
@@ -137,7 +137,7 @@ func TestIngredientRepository_GetBySpoonacularID_NotFound(t *testing.T) {
 	}
 }
 
-func TestIngredientRepository_ListUntranslated(t *testing.T) {
+func TestIngredientRepository_ListMissingTranslation(t *testing.T) {
 	pool := testutil.SetupTestDB(t)
 	repo := NewRepository(pool)
 	ctx := context.Background()
@@ -145,7 +145,7 @@ func TestIngredientRepository_ListUntranslated(t *testing.T) {
 	cat := "produce"
 	unit := "g"
 
-	// Insert 3 without translation
+	// Insert 3 without any translation.
 	for i, name := range []string{"carrot", "onion", "garlic"} {
 		id := 4000 + i
 		_, err := repo.Upsert(ctx, &IngredientMapping{
@@ -160,45 +160,38 @@ func TestIngredientRepository_ListUntranslated(t *testing.T) {
 		}
 	}
 
-	// Insert 1 with translation (shouldn't appear in untranslated list)
+	// Insert 1 and add a Russian translation — should not appear in the result.
 	id := 4100
-	ruName := "помидор"
-	withTranslation := &IngredientMapping{
-		CanonicalName:   "tomato",
-		CanonicalNameRu: &ruName,
-		SpoonacularID:   &id,
-		Aliases:         json.RawMessage(`[]`),
-		Category:        &cat,
-		DefaultUnit:     &unit,
-	}
-	saved, err := repo.Upsert(ctx, withTranslation)
+	saved, err := repo.Upsert(ctx, &IngredientMapping{
+		CanonicalName: "tomato",
+		SpoonacularID: &id,
+		Aliases:       json.RawMessage(`[]`),
+		Category:      &cat,
+		DefaultUnit:   &unit,
+	})
 	if err != nil {
-		t.Fatalf("upsert with translation: %v", err)
+		t.Fatalf("upsert tomato: %v", err)
 	}
-	// The upsert doesn't set canonical_name_ru because the UPDATE clause doesn't include it
-	// We need to manually set it after
-	if err := repo.UpdateTranslation(ctx, saved.ID, "помидор", []string{"помидор", "томат"}); err != nil {
-		t.Fatalf("update translation: %v", err)
+	if err := repo.UpsertTranslation(ctx, saved.ID, "ru", "помидор", json.RawMessage(`["помидор","томат"]`)); err != nil {
+		t.Fatalf("upsert translation: %v", err)
 	}
 
-	untranslated, err := repo.ListUntranslated(ctx, 10, 0)
+	missing, err := repo.ListMissingTranslation(ctx, "ru", 10, 0)
 	if err != nil {
-		t.Fatalf("list untranslated: %v", err)
+		t.Fatalf("list missing translation: %v", err)
 	}
 
-	// Should return the 3 without translation (carrot, onion, garlic)
-	// The translated tomato should not appear
-	for _, m := range untranslated {
+	for _, m := range missing {
 		if m.CanonicalName == "tomato" {
-			t.Error("translated ingredient should not appear in ListUntranslated")
+			t.Error("translated ingredient should not appear in ListMissingTranslation")
 		}
 	}
-	if len(untranslated) < 3 {
-		t.Errorf("expected at least 3 untranslated, got %d", len(untranslated))
+	if len(missing) < 3 {
+		t.Errorf("expected at least 3 untranslated, got %d", len(missing))
 	}
 }
 
-func TestIngredientRepository_UpdateTranslation(t *testing.T) {
+func TestIngredientRepository_UpsertTranslation(t *testing.T) {
 	pool := testutil.SetupTestDB(t)
 	repo := NewRepository(pool)
 	ctx := context.Background()
@@ -218,33 +211,29 @@ func TestIngredientRepository_UpdateTranslation(t *testing.T) {
 		t.Fatalf("upsert: %v", err)
 	}
 
-	err = repo.UpdateTranslation(ctx, saved.ID, "куриная грудка",
-		[]string{"куриная грудка", "куриное филе"})
+	err = repo.UpsertTranslation(ctx, saved.ID, "ru", "куриная грудка",
+		json.RawMessage(`["куриная грудка","куриное филе"]`))
 	if err != nil {
-		t.Fatalf("update translation: %v", err)
+		t.Fatalf("upsert translation: %v", err)
 	}
 
-	got, err := repo.GetByID(ctx, saved.ID)
+	// Retrieve with Russian context — CanonicalName should be the Russian name.
+	ruCtx := locale.WithLang(ctx, "ru")
+	got, err := repo.GetByID(ruCtx, saved.ID)
 	if err != nil {
 		t.Fatalf("get by id: %v", err)
 	}
-	if got.CanonicalNameRu == nil || *got.CanonicalNameRu != "куриная грудка" {
-		t.Errorf("expected canonical_name_ru='куриная грудка', got %v", got.CanonicalNameRu)
+	if got.CanonicalName != "куриная грудка" {
+		t.Errorf("expected CanonicalName='куриная грудка', got %q", got.CanonicalName)
 	}
 
-	var aliases []string
-	if err := json.Unmarshal(got.Aliases, &aliases); err != nil {
-		t.Fatalf("unmarshal aliases: %v", err)
+	// Retrieve with English context (default) — CanonicalName should be the English name.
+	enCtx := locale.WithLang(ctx, "en")
+	gotEn, err := repo.GetByID(enCtx, saved.ID)
+	if err != nil {
+		t.Fatalf("get by id (en): %v", err)
 	}
-
-	hasRu := false
-	for _, a := range aliases {
-		if a == "куриное филе" {
-			hasRu = true
-			break
-		}
-	}
-	if !hasRu {
-		t.Errorf("Russian alias not found in aliases: %v", aliases)
+	if gotEn.CanonicalName != "chicken_breast" {
+		t.Errorf("expected English CanonicalName='chicken_breast', got %q", gotEn.CanonicalName)
 	}
 }