fix(capture): derive deltacast audio PTS from sample count, not wall-clock

Removing -use_wallclock_as_timestamps on the SDI audio input. The bridge writes
SDI-clock-paced samples, so PTS from the 48kHz sample count shares the video's
clock domain and the audio length tracks the video length exactly. Wall-clock
timestamps made audio length = real elapsed time, which drifted ~1% longer than
the frame-count video when the encoder dipped under realtime (pitch-up).

services/capture/src/capture-manager.js

@@ -706,13 +706,18 @@ class CaptureManager {
           '-video_size', fcSize,
           '-framerate',  fcFps,
           '-i', 'pipe:0',
-          // Audio FIFO → ffmpeg input 1. Keep wallclock on audio so A/V sync
-          // aligns by arrival time; aresample=async=1 (applied on the master
-          // output) resamples audio to match the video CFR timestamps.
+          // Audio FIFO → ffmpeg input 1. The bridge writes EXACTLY the SDI-clock
+          // paced samples (group 0 is the reference, same slot clock as video),
+          // so we DERIVE audio PTS from the sample count at 48 kHz — NOT from
+          // wall-clock arrival. Wall-clock timestamping made the audio stream's
+          // length equal real elapsed time while video length = frame_count/fps;
+          // when the encoder ran a hair under realtime the audio ended up ~1%
+          // longer than video (heard as a pitch-up). Reading the raw stream at
+          // its natural rate keeps both in the same SDI clock domain; the
+          // master-output aresample=async=1 still soaks up any micro-jitter.
           // The FIFO carries the full 16ch the bridge publishes; channel
           // SELECTION (keep first N) is applied as an output filter so the
           // discrete broadcast channels are preserved, not downmixed.
-          '-use_wallclock_as_timestamps', '1',
           '-thread_queue_size', '512',
           '-f', 's16le',
           '-ar', '48000',