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"