Add only the track when a link carries both a video and a list

A link copied from an open mix (watch?v=…&list=RD…) expanded into the
whole radio station — 500 entries from one paste. Any URL with a `v=`
parameter now resolves to that single track, and only a /playlist?list=…
URL expands. The rule is deliberately blunt rather than keyed on
start_radio, so pasting a link behaves the same way every time.

Playlist expansion is also capped separately from the queue limit
(MAX_PLAYLIST_TRACKS, 100 by default) and the chat says when a playlist
hit that ceiling.

Verified against the real yt-dlp: both mix links resolve to one track, a
plain video link to one, and a playlist URL to its 13 entries.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.env.example

@@ -49,6 +49,9 @@ YTDLP_PATH=yt-dlp
 
 DEFAULT_VOLUME=60
 MAX_QUEUE_SIZE=500
+
+# Сколько треков максимум добавит одна ссылка на плейлист или микс.
+MAX_PLAYLIST_TRACKS=100
 SEARCH_RESULT_LIMIT=10
 
 # Через сколько секунд после ухода ПОСЛЕДНЕГО человека бот покидает голосовой