Fix quality: use tv_embedded,web player client instead of web-only

YouTube's web client gets SABR format restrictions in 2025-2026 yt-dlp,
limiting available streams and causing fallback to 360p. tv_embedded
bypasses SABR and exposes the full format list including 4K.

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

backend/services/ytdlp.py

@@ -232,7 +232,7 @@ 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=web",
+        "--extractor-args", "youtube:player_client=tv_embedded,web",
     ]
     stdout, stderr, code = _run([*base_cmd, *cookie_args], timeout=30)
     if code != 0:
@@ -646,7 +646,7 @@ def start_download(
                     "--no-part", "--no-mtime",
                     "-o", output_template,
                     "--newline", "--progress", "--no-colors",
-                    "--extractor-args", "youtube:player_client=web",
+                    "--extractor-args", "youtube:player_client=tv_embedded,web",
                     *cookie_args,
                 ],
                 stdout=subprocess.PIPE,