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/recipe/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"
 )
 
@@ -143,7 +144,7 @@ func TestRecipeRepository_GetByID_NotFound(t *testing.T) {
 	}
 }
 
-func TestRecipeRepository_ListUntranslated_Pagination(t *testing.T) {
+func TestRecipeRepository_ListMissingTranslation_Pagination(t *testing.T) {
 	pool := testutil.SetupTestDB(t)
 	repo := NewRepository(pool)
 	ctx := context.Background()
@@ -165,16 +166,16 @@ func TestRecipeRepository_ListUntranslated_Pagination(t *testing.T) {
 		}
 	}
 
-	untranslated, err := repo.ListUntranslated(ctx, 3, 0)
+	missing, err := repo.ListMissingTranslation(ctx, "ru", 3, 0)
 	if err != nil {
-		t.Fatalf("list untranslated: %v", err)
+		t.Fatalf("list missing translation: %v", err)
 	}
-	if len(untranslated) != 3 {
-		t.Errorf("expected 3 results with limit=3, got %d", len(untranslated))
+	if len(missing) != 3 {
+		t.Errorf("expected 3 results with limit=3, got %d", len(missing))
 	}
 }
 
-func TestRecipeRepository_UpdateTranslation(t *testing.T) {
+func TestRecipeRepository_UpsertTranslation(t *testing.T) {
 	pool := testutil.SetupTestDB(t)
 	repo := NewRepository(pool)
 	ctx := context.Background()
@@ -196,40 +197,52 @@ func TestRecipeRepository_UpdateTranslation(t *testing.T) {
 
 	titleRu := "Курица Тикка Масала"
 	descRu := "Классическое индийское блюдо"
-	stepsRu := json.RawMessage(`[{"number":1,"description":"Heat oil","description_ru":"Разогрейте масло"}]`)
+	stepsRu := json.RawMessage(`[{"number":1,"description":"Разогрейте масло"}]`)
 
-	if err := repo.UpdateTranslation(ctx, saved.ID, &titleRu, &descRu, stepsRu); err != nil {
-		t.Fatalf("update translation: %v", err)
+	if err := repo.UpsertTranslation(ctx, saved.ID, "ru", &titleRu, &descRu, nil, stepsRu); err != nil {
+		t.Fatalf("upsert translation: %v", err)
 	}
 
-	got, err := repo.GetByID(ctx, saved.ID)
+	// Retrieve with Russian context — title and steps should be translated.
+	ruCtx := locale.WithLang(ctx, "ru")
+	got, err := repo.GetByID(ruCtx, saved.ID)
 	if err != nil {
 		t.Fatalf("get by id: %v", err)
 	}
-	if got.TitleRu == nil || *got.TitleRu != titleRu {
-		t.Errorf("expected title_ru=%q, got %v", titleRu, got.TitleRu)
+	if got.Title != titleRu {
+		t.Errorf("expected title=%q, got %q", titleRu, got.Title)
 	}
-	if got.DescriptionRu == nil || *got.DescriptionRu != descRu {
-		t.Errorf("expected description_ru=%q, got %v", descRu, got.DescriptionRu)
+	if got.Description == nil || *got.Description != descRu {
+		t.Errorf("expected description=%q, got %v", descRu, got.Description)
 	}
 
 	var steps []RecipeStep
 	if err := json.Unmarshal(got.Steps, &steps); err != nil {
 		t.Fatalf("unmarshal steps: %v", err)
 	}
-	if len(steps) == 0 || steps[0].DescriptionRu == nil || *steps[0].DescriptionRu != "Разогрейте масло" {
-		t.Errorf("expected description_ru in steps, got %v", steps)
+	if len(steps) == 0 || steps[0].Description != "Разогрейте масло" {
+		t.Errorf("expected Russian step description, got %v", steps)
+	}
+
+	// Retrieve with English context — should return original English content.
+	enCtx := locale.WithLang(ctx, "en")
+	gotEn, err := repo.GetByID(enCtx, saved.ID)
+	if err != nil {
+		t.Fatalf("get by id (en): %v", err)
+	}
+	if gotEn.Title != "Chicken Tikka Masala" {
+		t.Errorf("expected English title, got %q", gotEn.Title)
 	}
 }
 
-func TestRecipeRepository_ListUntranslated_ExcludesTranslated(t *testing.T) {
+func TestRecipeRepository_ListMissingTranslation_ExcludesTranslated(t *testing.T) {
 	pool := testutil.SetupTestDB(t)
 	repo := NewRepository(pool)
 	ctx := context.Background()
 
 	diff := "easy"
 
-	// Insert untranslated
+	// Insert untranslated recipes.
 	for i := 0; i < 3; i++ {
 		spID := 60000 + i
 		_, err := repo.Upsert(ctx, &Recipe{
@@ -246,7 +259,7 @@ func TestRecipeRepository_ListUntranslated_ExcludesTranslated(t *testing.T) {
 		}
 	}
 
-	// Insert translated
+	// Insert one recipe and add a Russian translation.
 	spID := 60100
 	translated, err := repo.Upsert(ctx, &Recipe{
 		Source:        "spoonacular",
@@ -261,21 +274,21 @@ func TestRecipeRepository_ListUntranslated_ExcludesTranslated(t *testing.T) {
 		t.Fatalf("upsert translated: %v", err)
 	}
 	titleRu := "Переведённый рецепт"
-	if err := repo.UpdateTranslation(ctx, translated.ID, &titleRu, nil, translated.Steps); err != nil {
-		t.Fatalf("update translation: %v", err)
+	if err := repo.UpsertTranslation(ctx, translated.ID, "ru", &titleRu, nil, nil, nil); 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)
 	}
-	for _, r := range untranslated {
+	for _, r := range missing {
 		if r.Title == "Translated Recipe" {
-			t.Error("translated recipe should not appear in ListUntranslated")
+			t.Error("translated recipe 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 missing, got %d", len(missing))
 	}
 }