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/menu.go

@@ -13,6 +13,7 @@ type MenuRequest struct {
 	Restrictions      []string
 	CuisinePrefs      []string
 	AvailableProducts []string
+	Lang              string // ISO 639-1 target language code, e.g. "en", "ru"
 }
 
 // DayPlan is the AI-generated plan for a single day.
@@ -63,6 +64,7 @@ func (c *Client) GenerateMenu(ctx context.Context, req MenuRequest) ([]DayPlan,
 				CuisinePrefs:      req.CuisinePrefs,
 				Count:             7,
 				AvailableProducts: req.AvailableProducts,
+				Lang:              req.Lang,
 			})
 			results[idx] = mealResult{r, err}
 		}(i, slot.mealType, slot.fraction)