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/gemini/recipe.go

@@ -14,12 +14,13 @@ type RecipeGenerator interface {
 
 // RecipeRequest contains parameters for recipe generation.
 type RecipeRequest struct {
-	UserGoal          string   // "weight_loss" | "maintain" | "gain"
+	UserGoal          string   // "lose" | "maintain" | "gain"
 	DailyCalories     int
 	Restrictions      []string // e.g. ["gluten_free", "vegetarian"]
 	CuisinePrefs      []string // e.g. ["russian", "asian"]
 	Count             int
 	AvailableProducts []string // human-readable list of products in user's pantry
+	Lang              string   // ISO 639-1 target language code, e.g. "en", "ru"
 }
 
 // Recipe is a recipe returned by Gemini.
@@ -62,35 +63,55 @@ type NutritionInfo struct {
 	Approximate bool    `json:"approximate"`
 }
 
+// langNames maps ISO 639-1 codes to English language names used in the prompt.
+var langNames = map[string]string{
+	"en": "English",
+	"ru": "Russian",
+	"es": "Spanish",
+	"de": "German",
+	"fr": "French",
+	"it": "Italian",
+	"pt": "Portuguese",
+	"zh": "Chinese (Simplified)",
+	"ja": "Japanese",
+	"ko": "Korean",
+	"ar": "Arabic",
+	"hi": "Hindi",
+}
+
+// goalNames maps internal goal codes to English descriptions used in the prompt.
+var goalNames = map[string]string{
+	"lose":     "weight loss",
+	"maintain": "weight maintenance",
+	"gain":     "muscle gain",
+}
+
 // GenerateRecipes generates recipes using the Gemini AI.
 // Retries up to maxRetries times only when the response is not valid JSON.
 // API-level errors (rate limits, auth, etc.) are returned immediately.
 func (c *Client) GenerateRecipes(ctx context.Context, req RecipeRequest) ([]Recipe, error) {
 	prompt := buildRecipePrompt(req)
 
-	// OpenAI messages format.
 	messages := []map[string]string{
 		{"role": "user", "content": prompt},
 	}
 
 	var lastErr error
-	for attempt := 0; attempt < maxRetries; attempt++ {
+	for attempt := range maxRetries {
 		if attempt > 0 {
 			messages = []map[string]string{
 				{"role": "user", "content": prompt},
-				{"role": "user", "content": "Предыдущий ответ не был валидным JSON. Верни ТОЛЬКО массив JSON без какого-либо текста до или после."},
+				{"role": "user", "content": "Previous response was not valid JSON. Return ONLY a JSON array with no text before or after."},
 			}
 		}
 
 		text, err := c.generateContent(ctx, messages)
 		if err != nil {
-			// API-level error (4xx/5xx): no point retrying immediately.
 			return nil, err
 		}
 
 		recipes, err := parseRecipesJSON(text)
 		if err != nil {
-			// Malformed JSON from the model — retry with a clarifying message.
 			lastErr = fmt.Errorf("attempt %d parse JSON: %w", attempt+1, err)
 			continue
 		}
@@ -105,22 +126,26 @@ func (c *Client) GenerateRecipes(ctx context.Context, req RecipeRequest) ([]Reci
 }
 
 func buildRecipePrompt(req RecipeRequest) string {
-	goalRu := map[string]string{
-		"weight_loss": "похудение",
-		"maintain":    "поддержание веса",
-		"gain":        "набор массы",
+	lang := req.Lang
+	if lang == "" {
+		lang = "en"
 	}
-	goal := goalRu[req.UserGoal]
-	if goal == "" {
-		goal = "поддержание веса"
+	langName, ok := langNames[lang]
+	if !ok {
+		langName = "English"
 	}
 
-	restrictions := "нет"
+	goal := goalNames[req.UserGoal]
+	if goal == "" {
+		goal = "weight maintenance"
+	}
+
+	restrictions := "none"
 	if len(req.Restrictions) > 0 {
 		restrictions = strings.Join(req.Restrictions, ", ")
 	}
 
-	cuisines := "любые"
+	cuisines := "any"
 	if len(req.CuisinePrefs) > 0 {
 		cuisines = strings.Join(req.CuisinePrefs, ", ")
 	}
@@ -137,48 +162,46 @@ func buildRecipePrompt(req RecipeRequest) string {
 
 	productsSection := ""
 	if len(req.AvailableProducts) > 0 {
-		productsSection = "\nДоступные продукты (приоритет — скоро истекают ⚠):\n" +
-			strings.Join(req.AvailableProducts, "\n") +
-			"\nПредпочтительно использовать эти продукты в рецептах.\n"
+		productsSection = "\nAvailable products (⚠ = expiring soon, prioritise these):\n" +
+			strings.Join(req.AvailableProducts, "\n") + "\n"
 	}
 
-	return fmt.Sprintf(`Ты — диетолог-повар. Предложи %d рецептов на русском языке.
+	return fmt.Sprintf(`You are a chef and nutritionist. Generate %d recipes in %s.
 
-Профиль пользователя:
-- Цель: %s
-- Дневная норма калорий: %d ккал
-- Ограничения: %s
-- Предпочтения: %s
+User profile:
+- Goal: %s
+- Daily calories: %d kcal
+- Dietary restrictions: %s
+- Cuisine preferences: %s
 %s
-Требования к каждому рецепту:
-- Калорийность на порцию: не более %d ккал
-- Время приготовления: до 60 минут
-- Укажи КБЖУ на порцию (приблизительно)
+Requirements for each recipe:
+- Max %d kcal per serving
+- Total cooking time: max 60 minutes
+- Include approximate macros per serving
 
-ВАЖНО: поле image_query заполняй ТОЛЬКО на английском языке — оно используется для поиска фото.
+IMPORTANT:
+- All text fields (title, description, ingredient names, units, step descriptions, tags) MUST be in %s.
+- The "image_query" field MUST always be in English (it is used for stock-photo search).
 
-Верни ТОЛЬКО валидный JSON-массив без markdown-обёртки:
+Return ONLY a valid JSON array without markdown or extra text:
 [{
-  "title": "Название",
-  "description": "2-3 предложения",
+  "title": "...",
+  "description": "2-3 sentences",
   "cuisine": "russian|asian|european|mediterranean|american|other",
   "difficulty": "easy|medium|hard",
   "prep_time_min": 10,
   "cook_time_min": 20,
   "servings": 2,
-  "image_query": "chicken breast vegetables healthy (ENGLISH ONLY, used for photo search)",
-  "ingredients": [{"name": "Куриная грудка", "amount": 300, "unit": "г"}],
+  "image_query": "short English photo-search query",
+  "ingredients": [{"name": "...", "amount": 300, "unit": "..."}],
   "steps": [{"number": 1, "description": "...", "timer_seconds": null}],
-  "tags": ["высокий белок"],
-  "nutrition_per_serving": {
-    "calories": 420, "protein_g": 48, "fat_g": 12, "carbs_g": 18
-  }
-}]`, count, goal, req.DailyCalories, restrictions, cuisines, productsSection, perMealCalories)
+  "tags": ["..."],
+  "nutrition_per_serving": {"calories": 420, "protein_g": 48, "fat_g": 12, "carbs_g": 18}
+}]`, count, langName, goal, req.DailyCalories, restrictions, cuisines, productsSection, perMealCalories, langName)
 }
 
 func parseRecipesJSON(text string) ([]Recipe, error) {
 	text = strings.TrimSpace(text)
-	// Strip potential markdown code fences
 	if strings.HasPrefix(text, "```") {
 		text = strings.TrimPrefix(text, "```json")
 		text = strings.TrimPrefix(text, "```")