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/model.go

@@ -7,14 +7,15 @@ import (
 
 // IngredientMapping is the canonical ingredient record used to link
 // user products, recipe ingredients, and Spoonacular data.
+// CanonicalName holds the content for the language resolved at query time
+// (English by default, or from ingredient_translations when available).
 type IngredientMapping struct {
-	ID              string          `json:"id"`
-	CanonicalName   string          `json:"canonical_name"`
-	CanonicalNameRu *string         `json:"canonical_name_ru"`
-	SpoonacularID   *int            `json:"spoonacular_id"`
-	Aliases         json.RawMessage `json:"aliases"`    // []string
-	Category        *string         `json:"category"`
-	DefaultUnit     *string         `json:"default_unit"`
+	ID            string          `json:"id"`
+	CanonicalName string          `json:"canonical_name"`
+	SpoonacularID *int            `json:"spoonacular_id"`
+	Aliases       json.RawMessage `json:"aliases"`    // []string
+	Category      *string         `json:"category"`
+	DefaultUnit   *string         `json:"default_unit"`
 
 	CaloriesPer100g *float64 `json:"calories_per_100g"`
 	ProteinPer100g  *float64 `json:"protein_per_100g"`