fix(playout): re-encode HLS preview to uniform 2s segments so hls.js syncs

The audio-strip fix made the stream decode cleanly server-side, but the
browser monitor was still black: with -c:v copy the re-mux passed through
CasparCG's erratic real-time keyframes (segments 0.6-2.8s) and irregular
PTS. hls.js can't build a live timeline from that — it logs
"sliding 0.00 / prev-sn na / MISSED" and never loads a fragment (verified
live in-browser: readyState stays 0). A standalone ffmpeg honours
-force_key_frames, so re-encode to libx264 with a forced 2s GOP + CFR
fps=30, scaled to 720p for a light confidence monitor. Every HLS segment
is now a clean, keyframe-aligned 2.0s chunk hls.js can sync to.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/playout/src/playout-manager.js

@@ -221,9 +221,22 @@ export class PlayoutManager {
       '-fflags', '+genpts',
       '-analyzeduration', '2000000', '-probesize', '2000000',
       '-i', `${PREVIEW_UDP_URL}?fifo_size=1000000&overrun_nonfatal=1`,
-      // Drop the (broken) audio entirely; copy the clean video bitstream.
+      // Drop the (broken) audio entirely.
       '-an',
-      '-c:v', 'copy',
+      // Re-encode (NOT -c:v copy) to uniform, keyframe-aligned 2s segments with
+      // regenerated CFR timestamps. -c:v copy passed CasparCG's erratic
+      // real-time keyframes straight through, producing segments of 0.6–2.8s
+      // and irregular PTS; hls.js can't build a live timeline from that — it
+      // logs "sliding 0.00 / MISSED", never loads a fragment, and the monitor
+      // stays black even though the stream decodes cleanly server-side. A
+      // standalone ffmpeg honours -force_key_frames, so every GOP (and thus
+      // every HLS segment) is exactly 2.0s. fps=30 forces CFR; scale to 720p
+      // keeps the confidence monitor cheap.
+      '-vf', 'fps=30,scale=-2:720,format=yuv420p',
+      '-c:v', 'libx264', '-preset', 'veryfast', '-tune', 'zerolatency',
+      '-b:v', '1500k', '-maxrate', '2M', '-bufsize', '4M',
+      '-g', '60', '-keyint_min', '60', '-sc_threshold', '0',
+      '-force_key_frames', 'expr:gte(t,n_forced*2)',
       '-f', 'hls',
       '-hls_time', '2',
       '-hls_list_size', '8',