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/download.go

@@ -29,6 +29,7 @@ type stepState struct {
 	Container     string
 	FormatIDs     []string
 	PlaylistState *domain.PlaylistState
+	SearchState   *domain.SearchState
 }
 
 func (h *Handler) getUserStepState(chatID int64) *stepState {