Add Node.js to Docker image; use web player client only

Node.js is required by yt-dlp to solve YouTube's n-challenge (format URL
deobfuscation). Without it the web client returns no video formats.

The tv and ios player clients were removed — both require GVS PO tokens
that we don't have, so they only produce warnings and block every request.
The web client with Node.js installed gives 30+ formats and works cleanly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/ytdlp.py

@@ -216,7 +216,7 @@ def fetch_video_metadata(video_id: str) -> dict | None:
     base_cmd = [
         "yt-dlp", url,
         "--dump-json", "--no-download", "--no-playlist",
-        "--extractor-args", "youtube:player_client=tv,ios,web",
+        "--extractor-args", "youtube:player_client=web",
     ]
     stdout, stderr, code = _run([*base_cmd, *cookie_args], timeout=30)
     if code != 0:
@@ -567,7 +567,7 @@ def start_download(
                     "--no-part", "--no-mtime",
                     "-o", output_template,
                     "--newline", "--progress", "--no-colors",
-                    "--extractor-args", "youtube:player_client=tv,ios,web",
+                    "--extractor-args", "youtube:player_client=web",
                     *_cookie_args(),
                 ],
                 stdout=subprocess.PIPE,