Give yt-dlp a JavaScript runtime to avoid YouTube's bot checks

Downloads failed with "Sign in to confirm you're not a bot" even with
cookies and a working proxy. yt-dlp was also warning that no JavaScript
runtime was available: YouTube's player challenges now require one, and
without it extraction degrades into exactly those bot checks.

The image already ships Node, so yt-dlp is pointed at it via
--js-runtimes, but only after confirming the installed build understands
the flag — an older binary would otherwise fail on an unknown option.
Startup logs which runtime was picked and warns when there is none.

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

.env.example

@@ -57,6 +57,11 @@ YTDLP_PATH=yt-dlp
 # на хосте, используйте socks5h://host.docker.internal:1080 и добавьте в
 # compose.yml к extra_hosts строку "host.docker.internal:host-gateway".
 
+# JS-рантайм для yt-dlp: YouTube требует исполнять свой JS, иначе извлечение
+# деградирует и начинаются бот-проверки. В образе уже есть Node, поэтому по
+# умолчанию используется он. Пустое значение отключает флаг.
+YTDLP_JS_RUNTIME=node
+
 # Необязательно: дополнительные --extractor-args, через ";".
 # Помогает, когда YouTube не отдаёт форматы серверному IP:
 # YTDLP_EXTRACTOR_ARGS=youtube:player_client=default,web_safari