Anti-bot: configurable YouTube player clients (global + per-channel)

YouTube's bot-detection cracks down on different player clients over
time — the `web` client gets captcha-walled, then later `tv`/`mweb` are
the safe ones, etc. Let the user route around whichever client is
currently being targeted instead of being stuck with yt-dlp's pick.

- New backup.youtube_player_clients config (comma-separated, blank =
  yt-dlp default). Maps to --extractor-args youtube:player_client=…
- Per-channel DownloadOptions.youtube_player_clients override (None =
  defer to global). Resolved in the downloader's apply_player_client,
  which omits the flag entirely when empty so yt-dlp keeps its default
  client set (the recommended baseline).
- Threaded into the live downloader at construction + settings save
  (app + web).

UI: a "YouTube player clients" field in both global Settings (desktop +
web) and the per-channel options dialog, with hints pointing at
"tv,mweb" as the current least-bot-checked combo for captcha-prone
channels.

Verified the settings round-trip (global persists to config + echoes;
per-channel override saves + reads back). 87 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/web_ui/index.html

@@ -957,6 +957,10 @@ async function openChannelOptions(idx){
       ${triSelect('opt-subs-embed',opts.subtitles_embed)}</div>
     <div class="settings-row"><label>Convert format (blank = global)</label>
       <input type="text" id="opt-subs-format" value="${esc(opts.subtitle_format||'')}" placeholder="srt/vtt/ass" style="width:100px;background:var(--bg);color:var(--text);border:1px solid var(--border);border-radius:4px;padding:4px 6px"></div>
+    <div class="settings-row" style="flex-direction:column;align-items:stretch;gap:4px">
+      <label>YouTube player clients (blank = global)</label>
+      <input type="text" id="opt-player-clients" value="${esc(opts.youtube_player_clients||'')}" placeholder="e.g. tv,mweb" style="width:100%;background:var(--bg);color:var(--text);border:1px solid var(--border);border-radius:4px;padding:4px 6px"></div>
+    <div class="settings-hint" style="margin-bottom:4px">Per-channel <code>--extractor-args youtube:player_client</code> override. If this channel keeps hitting captchas, try <code>tv,mweb</code> (least bot-checked).</div>
     <div class="settings-row" style="flex-direction:column;align-items:stretch;gap:4px">
       <label>Extra yt-dlp args (one per line)</label>
       <textarea id="opt-extra" rows="3" placeholder="--no-mtime
@@ -990,6 +994,7 @@ function readChannelOptionsForm(){
     subtitles_auto:triValue('opt-subs-auto'),
     subtitles_embed:triValue('opt-subs-embed'),
     subtitle_format:strOrNull('opt-subs-format'),
+    youtube_player_clients:strOrNull('opt-player-clients'),
     extra_args:extra,
   };
 }
@@ -1459,6 +1464,11 @@ async function openSettings(){
       <span style="font-size:11px;color:var(--muted)">${cur.pot_provider_installed?'✓ installed':'not installed'}</span>
       <span id="pot-status" style="font-size:11px;color:var(--muted)"></span>
     </div>
+    <div class="settings-row" style="margin-top:8px">
+      <label for="cf-player-clients">YouTube player clients</label>
+      <input type="text" id="cf-player-clients" value="${esc(cur.youtube_player_clients||'')}" placeholder="default" style="width:140px;background:var(--bg);color:var(--text);border:1px solid var(--border);border-radius:4px;padding:4px 6px">
+    </div>
+    <div class="settings-hint" style="margin-bottom:6px">Comma-separated <code>--extractor-args youtube:player_client</code>. Blank = let yt-dlp pick (recommended). YouTube's bot-detection targets different clients over time; if you keep hitting captchas, try <code>tv,mweb</code> — currently the least-checked. Per-channel overrides live in each channel's options.</div>
     <hr style="border-color:var(--border);margin:12px 0">
     <div style="font-weight:700;margin-bottom:8px">Subtitles (global defaults)</div>
     <div class="settings-hint" style="margin-bottom:6px">Applied to every download. Individual channels can override these in their Channel options dialog.</div>
@@ -1658,7 +1668,8 @@ async function saveSettings(btn){
   const subsEmbed=document.getElementById('cf-subs-embed')?.checked||false;
   const subsLangs=document.getElementById('cf-subs-langs')?.value||'';
   const subsFormat=document.getElementById('cf-subs-format')?.value||'';
-  const payload={transcode,scheduler_enabled:schedEnabled,scheduler_interval_hours:schedInterval,max_concurrent:maxConcurrent,use_bundled_ytdlp:useBundledYtdlp,use_pot_provider:usePotProvider,subtitles_enabled:subsEnabled,subtitles_auto:subsAuto,subtitles_embed:subsEmbed,subtitle_langs:subsLangs,subtitle_format:subsFormat};
+  const playerClients=document.getElementById('cf-player-clients')?.value||'';
+  const payload={transcode,scheduler_enabled:schedEnabled,scheduler_interval_hours:schedInterval,max_concurrent:maxConcurrent,use_bundled_ytdlp:useBundledYtdlp,use_pot_provider:usePotProvider,subtitles_enabled:subsEnabled,subtitles_auto:subsAuto,subtitles_embed:subsEmbed,subtitle_langs:subsLangs,subtitle_format:subsFormat,youtube_player_clients:playerClients};
   if(bindMode)payload.bind_mode=bindMode;
   if(plexPath!==undefined)payload.plex_library_path=plexPath;
   if(sourceUrl!==undefined)payload.source_url=sourceUrl;