Add fast audio-only transcode mode; fixes no-sound from AC3/DTS/EAC3

Root cause of "movies have no volume control / can't hear anything":
0 of 200 movies had finished transcoding, so every movie streamed its
raw source file — and BDRip/AVI sources very commonly carry AC3/DTS/EAC3
audio, which no browser can decode natively. Browsers respond by hiding
the volume control entirely when they can't find a playable audio track,
which is exactly the symptom reported.

New "audio" transcode quality: video is stream-copied untouched, only
the audio track is re-encoded to AAC — dramatically faster than a full
ABR pass since no video re-encoding happens. Added:
- transcode_audio_fix() in transcode.py
- "audio" as a valid quality value everywhere quick/abr were validated
- POST /transcode/queue/fix-audio-all: cancels every still-pending job
  and replaces it with the fast audio fix for the same content (running
  jobs are left alone), so a library-wide backlog of slow ABR jobs can
  be swapped for something that actually restores sound soon
- Per-movie "Audio" button in Admin, "Fix Audio (Fast, All)" button on
  the Transcode Queue page

Note: this doesn't help HEVC/x265 sources, whose video (not just audio)
isn't natively browser-playable either — those still need a full ABR
re-encode to H.264.

frontend/src/pages/Admin.jsx

@@ -128,6 +128,10 @@ export default function Admin() {
                       title="Quick transcode (stream-copy, single bitrate)"
                       className="text-[10px] uppercase tracking-[0.2em] border border-[#222] hover:border-[#D9381E] hover:text-[#D9381E] text-[#8A8A8A] px-2 py-1 disabled:opacity-50"
                       data-testid={`transcode-quick-${m.id}`}>Quick</button>
+                    <button onClick={() => startTranscode(m, "audio")} disabled={transcoding[m.id]}
+                      title="Fix audio only (video untouched, audio re-encoded to AAC) — fast, fixes no-sound from AC3/DTS/EAC3"
+                      className="text-[10px] uppercase tracking-[0.2em] border border-[#222] hover:border-[#86efac] hover:text-[#86efac] text-[#8A8A8A] px-2 py-1 disabled:opacity-50"
+                      data-testid={`transcode-audio-${m.id}`}>Audio</button>
                     <button onClick={() => startTranscode(m, "abr")} disabled={transcoding[m.id]}
                       title="Adaptive bitrate (re-encodes to 360/480/720/1080p — slow)"
                       className="text-[10px] uppercase tracking-[0.2em] border border-[#222] hover:border-[#D9381E] hover:text-[#D9381E] text-[#8A8A8A] px-2 py-1 disabled:opacity-50"

frontend/src/pages/TranscodeQueue.jsx

@@ -1,7 +1,7 @@
 import { useEffect, useState } from "react";
 import api from "../lib/api";
 import { toast } from "sonner";
-import { RefreshCcw, X, RotateCcw, Pause, Play as PlayIcon, Trash2 } from "lucide-react";
+import { RefreshCcw, X, RotateCcw, Pause, Play as PlayIcon, Trash2, Volume2 } from "lucide-react";
 
 const STATUS_COLORS = {
   pending: "text-[#fcd34d]",
@@ -60,6 +60,17 @@ export default function TranscodeQueue() {
     } catch { toast.error("Could not toggle pause"); }
   };
 
+  const fixAudioAll = async () => {
+    if (!window.confirm(
+      "Cancels every job still pending (not yet started) and replaces it with a fast audio-only fix — video is left untouched, only the audio track is re-encoded to AAC. This is much faster than the full ABR queue and fixes \"no sound\" caused by AC3/DTS/EAC3 audio (common on BDRips), which browsers can't play natively. Continue?"
+    )) return;
+    try {
+      const { data } = await api.post("/transcode/queue/fix-audio-all");
+      toast.success(`Queued audio fix for ${data.queued} item(s), replacing ${data.cancelled} pending job(s)`);
+      load();
+    } catch (err) { toast.error(err.response?.data?.detail || "Could not start"); }
+  };
+
   const clearFinished = async () => {
     if (!window.confirm("Remove all done/failed/cancelled jobs from the history?")) return;
     try { const { data } = await api.post("/transcode/queue/clear"); toast.success(`Cleared ${data.deleted}`); load(); }
@@ -97,6 +108,9 @@ export default function TranscodeQueue() {
           <button onClick={retryAllFailed} disabled={!stats.failed && !stats.cancelled} className="flex items-center gap-2 border border-[#222] hover:border-[#D9381E] hover:text-[#D9381E] text-[#8A8A8A] px-4 py-2 text-xs uppercase tracking-[0.2em] transition-colors disabled:opacity-50" data-testid="queue-retry-all">
             <RotateCcw size={14} strokeWidth={1.5} /> Retry All Failed
           </button>
+          <button onClick={fixAudioAll} className="flex items-center gap-2 border border-[#222] hover:border-[#86efac] hover:text-[#86efac] text-[#8A8A8A] px-4 py-2 text-xs uppercase tracking-[0.2em] transition-colors" data-testid="queue-fix-audio-all">
+            <Volume2 size={14} strokeWidth={1.5} /> Fix Audio (Fast, All)
+          </button>
           <button onClick={clearFinished} className="flex items-center gap-2 border border-[#222] hover:border-[#fca5a5] hover:text-[#fca5a5] text-[#8A8A8A] px-4 py-2 text-xs uppercase tracking-[0.2em] transition-colors" data-testid="queue-clear">
             <Trash2 size={14} strokeWidth={1.5} /> Clear Finished
           </button>