thumbnails: stop video failures from re-logging every watcher tick

generate_video_thumbnail used .output().expect(...), which only catches
spawn failure — non-zero ffmpeg exits were silently discarded. With no
thumbnail and no .unsupported sentinel left behind, the watcher
re-detected the file as missing every quick-scan tick and re-logged
"New file detected (missing thumbnail)" forever.

Mirror the image branch: return io::Result, check status.success(),
and write the sentinel from create_thumbnails on failure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/video/actors.rs

@@ -107,19 +107,27 @@ pub async fn create_playlist(video_path: &str, playlist_file: &str) -> Result<Ch
     result
 }
 
-pub fn generate_video_thumbnail(path: &Path, destination: &Path) {
-    Command::new("ffmpeg")
+pub fn generate_video_thumbnail(path: &Path, destination: &Path) -> std::io::Result<()> {
+    let output = Command::new("ffmpeg")
         .arg("-ss")
         .arg("3")
         .arg("-i")
-        .arg(path.to_str().unwrap())
+        .arg(path)
         .arg("-vframes")
         .arg("1")
         .arg("-f")
         .arg("image2")
         .arg(destination)
-        .output()
-        .expect("Failure to create video frame");
+        .output()?;
+
+    if !output.status.success() {
+        return Err(std::io::Error::other(format!(
+            "ffmpeg failed ({}): {}",
+            output.status,
+            String::from_utf8_lossy(&output.stderr).trim()
+        )));
+    }
+    Ok(())
 }
 
 /// Use ffmpeg to extract a 200px-wide thumbnail from formats the `image` crate