fix: use GetLanguage helper in MultiSearch endpoint

Problem:
- MultiSearch endpoint was reading 'language' parameter only
- Frontend sends 'lang' parameter (from interceptor)
- Search results were always in Russian

Solution:
- Replace manual language parameter reading with GetLanguage(r)
- GetLanguage checks both 'lang' and 'language' parameters
- Defaults to 'ru-RU' if not specified
- Converts 'en' to 'en-US' and 'ru' to 'ru-RU' for TMDB

Before:
language := r.URL.Query().Get("language")
if language == "" {
    language = "ru-RU"
}

After:
language := GetLanguage(r)

Result:
✅ Search results now respect ?lang=en parameter
✅ Movie/TV titles in search are localized
✅ Consistent with other endpoints (movies, tv, etc.)

pkg/handlers/search.go

@@ -26,10 +26,7 @@ func (h *SearchHandler) MultiSearch(w http.ResponseWriter, r *http.Request) {
 	}
 
 	page := getIntQuery(r, "page", 1)
-	language := r.URL.Query().Get("language")
-	if language == "" {
-		language = "ru-RU"
-	}
+	language := GetLanguage(r)
 
 	results, err := h.tmdbService.SearchMulti(query, page, language)
 	if err != nil {