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/menu/handler.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/food-ai/backend/internal/gemini"
+	"github.com/food-ai/backend/internal/locale"
 	"github.com/food-ai/backend/internal/middleware"
 	"github.com/food-ai/backend/internal/savedrecipe"
 	"github.com/food-ai/backend/internal/user"
@@ -128,7 +129,7 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	menuReq := buildMenuRequest(u)
+	menuReq := buildMenuRequest(u, locale.FromContext(r.Context()))
 
 	// Attach pantry products.
 	if products, err := h.productLister.ListForPrompt(r.Context(), userID); err == nil {
@@ -460,8 +461,8 @@ type userPreferences struct {
 	Restrictions []string `json:"restrictions"`
 }
 
-func buildMenuRequest(u *user.User) gemini.MenuRequest {
-	req := gemini.MenuRequest{DailyCalories: 2000}
+func buildMenuRequest(u *user.User, lang string) gemini.MenuRequest {
+	req := gemini.MenuRequest{DailyCalories: 2000, Lang: lang}
 	if u.Goal != nil {
 		req.UserGoal = *u.Goal
 	}