fix(capture): align A/V at record start (kill leading silence + length drift)

Root cause of 'silent first ~1s then clean' + ~0.5% audio-too-long: in standby
the bridge keeps filling the audio FIFO while the idle-preview consumes only
video, so when recording starts ffmpeg reads a ~0.5s backlog of stale audio,
AND the video-only pre-roll discards video frames the audio never had.

Fix: (1) skip the video-only pre-roll in standby (warm slot = no unstable
frames), (2) drain the audio FIFO non-blocking immediately before ffmpeg opens
it, so audio starts at the live edge aligned with the first real video frame.

services/capture/src/capture-manager.js

@@ -732,7 +732,7 @@ class CaptureManager {
         ],
         isNetwork:      false,
         bridgeProcess:  fcPipeProcess,  /* capture-manager pipes this to ffmpeg stdin */
-        audioFifo:      null,
+        audioFifo:      audioFifoPath,  /* flushed just before ffmpeg opens it (A/V align) */
         interlaced:     fcInterlaced,
         audioInputIndex: 1,             /* audio FIFO is ffmpeg input 1 */
         _fcPipeProcess: fcPipeProcess,  /* stored for clean stop */
@@ -1030,16 +1030,47 @@ exit "$BMXRC"
       sourceType, sourceBackend, device, port, board, sourceUrl, listen, listenPort, streamKey,
     });
 
-    // ── Pre-roll: discard initial unstable frames ────────────────────────────
-    if (bridgeProcess && (sourceType === 'deltacast' || sourceType === 'blackmagic' || sourceType === 'sdi')) {
+    // ── Pre-roll + A/V alignment ─────────────────────────────────────────────
+    // The pre-roll drains the VIDEO pipe (fc_pipe) to discard unstable startup
+    // frames. In STANDBY the framecache slot is already warm, so there are no
+    // unstable frames — skip the video drain (draining only video while audio
+    // keeps buffering is exactly what offset the streams, giving "silent first
+    // second then clean").
+    if (bridgeProcess && !_standbyMode
+        && (sourceType === 'deltacast' || sourceType === 'blackmagic' || sourceType === 'sdi')) {
       console.log(`[capture] pre-rolling: discarding ${PRE_ROLL_SECONDS}s of frames`);
-      // Attach temporary drain listener.
-      bridgeProcess.stdout.on('data', () => {}); 
+      bridgeProcess.stdout.on('data', () => {});
       await new Promise(r => setTimeout(r, PRE_ROLL_SECONDS * 1000));
       bridgeProcess.stdout.removeAllListeners('data');
       console.log(`[capture] pre-roll complete.`);
     }
 
+    // FLUSH STALE AUDIO immediately before ffmpeg opens the FIFO. During standby
+    // the bridge keeps writing audio into the named FIFO while the idle-preview
+    // consumes only video, so the FIFO holds up to a full pipe buffer (~0.5s) of
+    // stale audio. Draining it here (right before the record ffmpeg attaches)
+    // makes audio start at the live edge, time-aligned with the first video
+    // frame — eliminating the leading silence + the ~0.5% audio-length surplus.
+    if (audioFifo) {
+      try {
+        const fsSync = await import('node:fs');
+        const fd = fsSync.openSync(audioFifo, fsSync.constants.O_RDONLY | fsSync.constants.O_NONBLOCK);
+        const tmp = Buffer.allocUnsafe(1 << 20);
+        let drained = 0;
+        for (;;) {
+          let n = 0;
+          try { n = fsSync.readSync(fd, tmp, 0, tmp.length, null); }
+          catch (e) { if (e.code === 'EAGAIN') break; throw e; }
+          if (n <= 0) break;
+          drained += n;
+        }
+        fsSync.closeSync(fd);
+        console.log(`[capture] flushed ${drained} bytes of stale standby audio before record`);
+      } catch (e) {
+        console.warn(`[capture] audio FIFO pre-flush skipped: ${e.message}`);
+      }
+    }
+
     const startedAt = new Date().toISOString();
     const recordingStartedAt = Date.now();