fix(capture): skip video-only pre-roll in standby to stop A/V pitch drift

The pre-roll drained only the video pipe (fc_pipe) while the audio FIFO kept
buffering, so ffmpeg read ~PRE_ROLL_SECONDS of surplus pre-roll audio — making
audio longer than video, which when synced compresses audio ~0.5% (pitch-up,
measured: 2591573 audio samples vs 2579395 expected for the video duration).

In standby the framecache slot is already warm (no unstable startup frames), so
the drain is unnecessary; skipping it lets ffmpeg open video and audio together
from the same instant. Cold on-demand spawns keep the brief drain.

services/capture/src/capture-manager.js

@@ -1078,13 +1078,26 @@ exit "$BMXRC"
     if (sourceAudioChannels && wantAudioChannels) audioChannels = effAudioChannels;
 
     // ── Pre-roll: discard initial unstable frames ────────────────────────────
-    if (bridgeProcess && (sourceType === 'deltacast' || sourceType === 'blackmagic' || sourceType === 'sdi')) {
+    // CRITICAL A/V-SYNC NOTE: this drains ONLY the VIDEO pipe (fc_pipe stdout).
+    // The audio FIFO is opened by ffmpeg, not here, so it keeps BUFFERING during
+    // the drain — when ffmpeg starts it reads that buffered pre-roll audio,
+    // making the audio stream ~PRE_ROLL_SECONDS longer than video. Synced to the
+    // video length that surplus audio is compressed → a slight pitch-up.
+    //
+    // In STANDBY mode the framecache slot has been warm for a long time, so there
+    // are NO unstable startup frames to discard — skip the asymmetric drain
+    // entirely and let ffmpeg open video (fc_pipe) and audio (FIFO) together so
+    // both streams start from the same instant. Only the legacy on-demand spawn
+    // (cold slot) still needs the brief drain.
+    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.`);
+    } else if (bridgeProcess) {
+      console.log('[capture] standby/warm slot — skipping video-only pre-roll to keep A/V aligned');
     }
 
     const startedAt = new Date().toISOString();