feat: add ?lang=en support to API with default ru

Language Support:
- Create GetLanguage() helper function
- Support both 'lang' and 'language' query parameters
- Convert short codes (en/ru) to TMDB format (en-US/ru-RU)
- Default language: ru-RU

Changes:
- pkg/handlers/lang_helper.go: new helper for language detection
- pkg/handlers/movie.go: use GetLanguage() in all endpoints
- pkg/handlers/tv.go: use GetLanguage() in all endpoints

How to use:
- ?lang=en → returns English content
- ?lang=ru → returns Russian content (default)
- No param → defaults to Russian

All movie/TV endpoints now support multilingual content:
GET /api/v1/movies/{id}?lang=en
GET /api/v1/movies/popular?lang=en
GET /api/v1/tv/{id}?lang=en
etc.

pkg/handlers/lang_helper.go

@@ -0,0 +1,35 @@
+package handlers
+
+import (
+	"net/http"
+)
+
+// GetLanguage extracts the lang parameter from request and returns it with default "ru"
+// Supports both "lang" and "language" query parameters
+// Valid values: "ru", "en"
+// Default: "ru"
+func GetLanguage(r *http.Request) string {
+	// Check "lang" parameter first (our new standard)
+	lang := r.URL.Query().Get("lang")
+	
+	// Fall back to "language" for backward compatibility
+	if lang == "" {
+		lang = r.URL.Query().Get("language")
+	}
+	
+	// Default to "ru" if not specified
+	if lang == "" {
+		return "ru-RU"
+	}
+	
+	// Convert short codes to TMDB format
+	switch lang {
+	case "en":
+		return "en-US"
+	case "ru":
+		return "ru-RU"
+	default:
+		// Return as-is if already in correct format
+		return lang
+	}
+}