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>

src/sources/ytdlp.ts

@@ -31,8 +31,28 @@ interface YtDlpEntry {
   entries?: YtDlpEntry[] | null;
 }
 
+/**
+ * Modern yt-dlp needs a JavaScript runtime for YouTube's player challenges;
+ * without one, extraction is degraded and YouTube starts answering with "Sign in
+ * to confirm you're not a bot". Our image already ships Node, so we point yt-dlp
+ * at it — but only once we have confirmed this build understands the flag.
+ */
+let jsRuntimeArgs: string[] = [];
+
+async function detectJsRuntime(): Promise<string | null> {
+  if (!config.YTDLP_JS_RUNTIME) return null;
+  try {
+    const { stdout } = await runYtDlp(["--help"], 15_000);
+    if (!stdout.includes("--js-runtimes")) return null;
+    jsRuntimeArgs = ["--js-runtimes", config.YTDLP_JS_RUNTIME];
+    return config.YTDLP_JS_RUNTIME;
+  } catch {
+    return null;
+  }
+}
+
 function baseArgs(): string[] {
-  const args = ["--no-warnings", "--no-playlist-reverse", "--ignore-config", "--no-color"];
+  const args = ["--no-warnings", "--no-playlist-reverse", "--ignore-config", "--no-color", ...jsRuntimeArgs];
   if (config.YTDLP_COOKIES) args.push("--cookies", config.YTDLP_COOKIES);
   if (config.YTDLP_PROXY) args.push("--proxy", config.YTDLP_PROXY);
   for (const value of config.YTDLP_EXTRACTOR_ARGS?.split(";") ?? []) {
@@ -351,10 +371,17 @@ export function openAudioStream(pageUrl: string): AudioProcess {
   };
 }
 
-export async function checkAvailable(): Promise<string | null> {
+export interface YtDlpStatus {
+  version: string;
+  jsRuntime: string | null;
+}
+
+export async function checkAvailable(): Promise<YtDlpStatus | null> {
   try {
     const { stdout } = await runYtDlp(["--version"], 15_000);
-    return stdout.trim() || null;
+    const version = stdout.trim();
+    if (!version) return null;
+    return { version, jsRuntime: await detectJsRuntime() };
   } catch {
     return null;
   }