chore(video): drop unused Ffmpeg::_generate_playlist, fix reels comment

_generate_playlist was a private, never-called predecessor of the HLS
path in video/actors.rs. Its leftover args (-hls_list_size 100, width
1080 scale, libx264 only) no longer described how playlists are built.
Removing it also drops the now-unused std::process::Output import.

The reels encoder comment said its NVENC settings matched the HLS
transcode; reels use preset p4, HLS uses p1. Only the quality target
(cq 23) is shared.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/reels/render.rs

@@ -155,7 +155,8 @@ pub fn beat_filtergraph(opts: &SegmentOpts, per_photo: &[f64]) -> String {
 
 fn video_encoder_args(nvenc: bool) -> Vec<String> {
     if nvenc {
-        // p4 ≈ balanced; cq 23 ≈ libx264 crf 21. Matches the HLS transcode path.
+        // p4 ≈ balanced; cq 23 ≈ libx264 crf 21. Same quality target as the
+        // HLS transcode, which uses the faster p1 preset for throughput.
         [
             "-c:v",
             "h264_nvenc",