fix: pass target audio codec to audio channel filter rather than source

Fixes issues with AAC output >2 channels

server/src/ffmpeg/builder/options/AudioOutputOptions.ts

@@ -1,3 +1,4 @@
+import { AudioFormats } from '../constants.ts';
 import { makeConstantOutputOption } from './OutputOption.ts';
 
 export const AudioChannelsOutputOption = (
@@ -8,7 +9,7 @@ export const AudioChannelsOutputOption = (
   const opts: string[] = [];
   if (
     sourceChannels !== desiredChannels ||
-    (audioFormat === 'aac' && desiredChannels > 2)
+    (audioFormat === AudioFormats.Aac && desiredChannels > 2)
   ) {
     opts.push('-ac', desiredChannels.toString());
   }

server/src/ffmpeg/builder/pipeline/BasePipelineBuilder.ts

@@ -580,7 +580,7 @@ export abstract class BasePipelineBuilder implements PipelineBuilder {
     if (!isNull(this.context.desiredAudioState.audioChannels)) {
       this.pipelineSteps.push(
         AudioChannelsOutputOption(
-          this.context.audioStream.codec,
+          this.context.desiredAudioState.audioEncoder,
           this.context.audioStream.channels,
           this.context.desiredAudioState.audioChannels,
         ),