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/recognition/handler.go

@@ -17,6 +17,7 @@ import (
 type ingredientRepo interface {
 	FuzzyMatch(ctx context.Context, name string) (*ingredient.IngredientMapping, error)
 	Upsert(ctx context.Context, m *ingredient.IngredientMapping) (*ingredient.IngredientMapping, error)
+	UpsertTranslation(ctx context.Context, id, lang, name string, aliases json.RawMessage) error
 }
 
 // Handler handles POST /ai/* recognition endpoints.
@@ -218,7 +219,6 @@ func (h *Handler) saveClassification(ctx context.Context, c *gemini.IngredientCl
 
 	m := &ingredient.IngredientMapping{
 		CanonicalName:   c.CanonicalName,
-		CanonicalNameRu: &c.CanonicalNameRu,
 		Category:        strPtr(c.Category),
 		DefaultUnit:     strPtr(c.DefaultUnit),
 		CaloriesPer100g: c.CaloriesPer100g,
@@ -234,6 +234,13 @@ func (h *Handler) saveClassification(ctx context.Context, c *gemini.IngredientCl
 		slog.Warn("upsert classified ingredient", "name", c.CanonicalName, "err", err)
 		return nil
 	}
+
+	// Persist the Russian translation when Gemini provided one.
+	if c.CanonicalNameRu != "" {
+		if err := h.ingredientRepo.UpsertTranslation(ctx, saved.ID, "ru", c.CanonicalNameRu, json.RawMessage("[]")); err != nil {
+			slog.Warn("upsert ingredient translation", "id", saved.ID, "err", err)
+		}
+	}
 	return saved
 }