feat(extraction): add audio_ready event emission

- Add EventEmitter to emit audio_ready when extraction completes
- Pass sessionId and audioPath in event data
- Export audioEvents for Main process orchestrator

Refs: S3-06 AC1,AC3,AC7

services/modules/extraction/ffmpegExtractor.js

@@ -1,3 +1,5 @@
+const EventEmitter = require('events');
+const audioEvents = new EventEmitter();
 
 // Ensure ffmpeg binary is available
 if (!ffmpegPath) {
@@ -75,6 +77,12 @@ module.exports = {
                     progressBar.update(100, { timemark: 'done' });
                     progressBar.stop();
                     console.log(`Extraction completed: ${outputAudioPath}`);
+
+                     audioEvents.emit('audio_ready', {
+                        sessionId: inputVideoName,
+                        audioPath: outputAudioPath
+                    });
+
                     resolve();
                 })
                 .on('error', (err) => {
@@ -88,6 +96,6 @@ module.exports = {
                 console.log();
             }
         });
-    }
-
+    },
+    audioEvents: audioEvents
 }