feat: add in-memory media info cache and YouTube search feature

- Add in-memory LRU cache (200 entries, 30 min TTL) to yt-dlp Client for
  GetMediaInfo and GetPlaylistInfo results, avoiding redundant yt-dlp calls
  for the same URL within a session
- Add YouTube (/search) and YouTube Music (/music) search with paginated
  results (5 per page), user selects a result to start the download flow
- Add SearchState type, new search.go handler, search results keyboard with
  < > navigation and New Search button
- Add Search button to main menu keyboard
- Update help text with new commands

internal/handler/keyboard.go

@@ -14,6 +14,7 @@ func mainKeyboard() models.InlineKeyboardMarkup {
 		InlineKeyboard: [][]models.InlineKeyboardButton{
 			{
 				{Text: "Download", CallbackData: "download"},
+				{Text: "Search", CallbackData: "search"},
 				{Text: "Settings", CallbackData: "settings"},
 			},
 			{