Add series metadata tagging and tests

Include series and seriesPart metadata when tagging audio files. For m4b output the code uses show and episode_id; for mp3 and flac it writes SERIES and SERIES-PART. Adds unit tests verifying tag output for .m4b, .mp3, and .flac and that tags are omitted when fields are absent.

src/lib/utils/metadata-tagger.ts

@@ -118,6 +118,14 @@ export async function tagAudioFileMetadata(
         args.push('-metadata', `ASIN="${escapeMetadata(metadata.asin)}"`);
       }
 
+      if (metadata.series) {
+        args.push('-metadata', `SERIES="${escapeMetadata(metadata.series)}"`);
+      }
+
+      if (metadata.seriesPart) {
+        args.push('-metadata', `SERIES-PART="${escapeMetadata(metadata.seriesPart)}"`);
+      }
+
       // Explicitly specify output format
       args.push('-f', 'flac');
     }