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/recommendation/handler.go

@@ -9,6 +9,7 @@ import (
 	"sync"
 
 	"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/user"
 )
@@ -74,7 +75,7 @@ func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	req := buildRecipeRequest(u, count)
+	req := buildRecipeRequest(u, count, locale.FromContext(r.Context()))
 
 	// Attach available products to personalise the prompt.
 	if products, err := h.productLister.ListForPrompt(r.Context(), userID); err == nil {
@@ -108,10 +109,11 @@ func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, recipes)
 }
 
-func buildRecipeRequest(u *user.User, count int) gemini.RecipeRequest {
+func buildRecipeRequest(u *user.User, count int, lang string) gemini.RecipeRequest {
 	req := gemini.RecipeRequest{
 		Count:         count,
 		DailyCalories: 2000, // sensible default
+		Lang:          lang,
 	}
 
 	if u.Goal != nil {