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/video/ffmpeg.rs

@@ -1,7 +1,7 @@
 use futures::TryFutureExt;
 use log::{debug, error, info, warn};
 use std::io::Result;
-use std::process::{Output, Stdio};
+use std::process::Stdio;
 use std::sync::OnceLock;
 use std::time::Instant;
 use tokio::process::Command;
@@ -47,37 +47,6 @@ pub enum GifType {
 }
 
 impl Ffmpeg {
-    async fn _generate_playlist(&self, input_file: &str, output_file: &str) -> Result<String> {
-        let ffmpeg_result: Result<Output> = Command::new("ffmpeg")
-            .arg("-i")
-            .arg(input_file)
-            .arg("-c:v")
-            .arg("h264")
-            .arg("-crf")
-            .arg("21")
-            .arg("-preset")
-            .arg("veryfast")
-            .arg("-hls_time")
-            .arg("3")
-            .arg("-hls_list_size")
-            .arg("100")
-            .arg("-vf")
-            .arg("scale=1080:-2,setsar=1:1")
-            .arg(output_file)
-            .stdout(Stdio::null())
-            .stderr(Stdio::piped())
-            .output()
-            .inspect_err(|e| error!("Failed to run ffmpeg on child process: {}", e))
-            .map_err(|e| std::io::Error::other(e.to_string()))
-            .await;
-
-        if let Ok(ref res) = ffmpeg_result {
-            debug!("ffmpeg output: {:?}", res);
-        }
-
-        ffmpeg_result.map(|_| output_file.to_string())
-    }
-
     async fn get_video_duration(&self, input_file: &str) -> Result<u32> {
         Command::new("ffprobe")
             .args(["-i", input_file])