Run pyannote diarization in background thread with progress reporting

Move the blocking pipeline() call to a daemon thread and emit estimated
progress messages every 2 seconds from the main thread. The progress
estimate uses audio duration to calibrate the expected total time.
Also pass audio_duration_sec from PipelineService to DiarizeService.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

python/voice_to_notes/services/pipeline.py

@@ -121,6 +121,7 @@ class PipelineService:
             num_speakers=num_speakers,
             min_speakers=min_speakers,
             max_speakers=max_speakers,
+            audio_duration_sec=transcription.duration_ms / 1000.0,
         )
 
         # Step 3: Merge