thumbnails: align video ffmpeg args with the image path so non-yuvj420p sources work

The bare 'ffmpeg -ss 3 -i in -vframes 1 -f image2 out' command failed on
sources whose decoded pix_fmt isn't yuvj420p (e.g. older Samsung phone
videos in yuv420p). With no -vf filter chain, the decoded frame goes
straight to the mjpeg encoder, which rejects it with 'Non full-range
YUV is non-standard' and exits non-zero.

generate_image_thumbnail_ffmpeg already handles the same class of
source for HEIC/RAW by adding -vf scale=200:-1 -c:v mjpeg — the filter
chain lets ffmpeg auto-insert the pix_fmt converter the encoder needs.
Adopt the same args here. Side benefit: video thumbnails are now 200px
wide on disk, matching image thumbnails (previously full-resolution).

Pre-existing .unsupported sentinels for videos that hit this failure
will need to be deleted manually to retry — they're under
$THUMBNAILS/<lib_id>/.../*.unsupported.

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

src/video/actors.rs

@@ -108,15 +108,27 @@ pub async fn create_playlist(video_path: &str, playlist_file: &str) -> Result<Ch
 }
 
 pub fn generate_video_thumbnail(path: &Path, destination: &Path) -> std::io::Result<()> {
+    // -vf scale + -c:v mjpeg mirrors `generate_image_thumbnail_ffmpeg`. The
+    // filter chain matters as much as the scale does: without it, ffmpeg
+    // hands the decoded frame straight to the mjpeg encoder, which rejects
+    // any non-yuvj420p source ("Non full-range YUV is non-standard"). The
+    // filter chain lets ffmpeg auto-insert the pix_fmt converter the
+    // encoder needs, which is how the image-thumbnail path already handles
+    // the same class of source.
     let output = Command::new("ffmpeg")
+        .arg("-y")
         .arg("-ss")
         .arg("3")
         .arg("-i")
         .arg(path)
         .arg("-vframes")
         .arg("1")
+        .arg("-vf")
+        .arg("scale=200:-1")
         .arg("-f")
         .arg("image2")
+        .arg("-c:v")
+        .arg("mjpeg")
         .arg(destination)
         .output()?;