fix: Configure MediaRecorder with consistent WebM/Opus format

- Fix root cause of "Thank you very much" repetitive transcriptions
- Configure MediaRecorder with audio/webm;codecs=opus for consistent encoding
- Add 128kbps audio bitrate for good quality
- Prevent "EBML header parsing failed" errors in STT service
- Follow unmute.sh methodology for WebM format consistency
- Add fallback handling for browsers without WebM/Opus support

Resolves STT service logs showing corrupted WebM files causing
ffmpeg conversion failures and silence fallback transcriptions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

webrtc/server/fastapi_integration.py

@@ -205,7 +205,18 @@ def create_fastapi_app() -> FastAPI:
                 console.log('Microphone access granted');
                 addTranscription('Microphone access granted', new Date().toISOString());
                 
-                mediaRecorder = new MediaRecorder(audioStream);
+                // Configure MediaRecorder with consistent WebM/Opus format (unmute.sh methodology)
+                const mimeType = 'audio/webm;codecs=opus';
+                if (!MediaRecorder.isTypeSupported(mimeType)) {
+                    console.warn('WebM/Opus not supported, falling back to default format');
+                    mediaRecorder = new MediaRecorder(audioStream);
+                } else {
+                    console.log('Using WebM/Opus format for consistent encoding');
+                    mediaRecorder = new MediaRecorder(audioStream, {
+                        mimeType: mimeType,
+                        audioBitsPerSecond: 128000  // 128 kbps for good quality
+                    });
+                }
                 
                 mediaRecorder.ondataavailable = function(event) {
                     console.log('Audio chunk available, size:', event.data.size);