Run background ffmpeg tasks as ProcessPriorityClass.BelowNormal

Follow TrickPlay example of running other background ffmpeg tasks as ProcessPriorityClass.BelowNormal:

- Keyframe extraction
- Media info probing during library scans
- Audio normalization
- Image extraction

src/Jellyfin.MediaEncoding.Keyframes/FfProbe/FfProbeKeyframeExtractor.cs

@@ -42,6 +42,7 @@ public static class FfProbeKeyframeExtractor
         try
         {
             process.Start();
+            process.PriorityClass = ProcessPriorityClass.BelowNormal;
 
             return ParseStream(process.StandardOutput);
         }