fix: Update Piper TTS to support voice preview and improve setup

- Fixed preview endpoint to work with both Kokoro and Piper engines
- Improved voice model download with curl fallback and SSL handling
- Added better error messages when models are missing
- Created PIPER_SETUP.md with manual download instructions
- Added test_piper_direct.py for testing Piper installation
- Updated README with voice model download requirements

Note: Piper voice models require manual download from GitHub releases
due to HuggingFace URL structure. See PIPER_SETUP.md for instructions.

backend/main.py

@@ -385,26 +385,42 @@ async def preview_voice(request: VoicePreviewRequest):
     preview_text = request.text or "Hello! This is a preview of this voice. How does it sound?"
     
     try:
-        # Get pipeline for language
-        pipeline = tts_service.get_pipeline(request.lang)
-        
-        # Generate preview audio
-        audio = tts_service.synthesize_segment(
-            pipeline=pipeline,
-            text=preview_text,
-            voice=request.voice,
-            speed=request.speed
-        )
-        
-        # Convert to WAV bytes
         import io
         import soundfile as sf
+        import numpy as np
+        
+        # Check which engine is active
+        if tts_service.engine_name == 'kokoro':
+            # Use Kokoro service
+            pipeline = tts_service.kokoro_service.get_pipeline(request.lang)
+            audio = tts_service.kokoro_service.synthesize_segment(
+                pipeline=pipeline,
+                text=preview_text,
+                voice=request.voice,
+                speed=request.speed
+            )
+            sample_rate = tts_service.kokoro_service.SAMPLE_RATE
+            
+        elif tts_service.engine_name == 'piper':
+            # Use Piper service with pitch control
+            audio = tts_service.piper_service.synthesize_with_pitch(
+                text=preview_text,
+                voice=request.voice,
+                speed=request.speed,
+                pitch_shift=0  # No pitch shift for preview
+            )
+            sample_rate = tts_service.piper_service.SAMPLE_RATE
+            
+        else:
+            raise ValueError(f"Unknown engine: {tts_service.engine_name}")
+        
+        # Convert to WAV bytes
         buffer = io.BytesIO()
-        sf.write(buffer, audio, tts_service.SAMPLE_RATE, format='WAV')
+        sf.write(buffer, audio, sample_rate, format='WAV')
         buffer.seek(0)
         audio_bytes = buffer.read()
         
-        logger.info(f"Generated voice preview: {len(audio_bytes)} bytes")
+        logger.info(f"Generated voice preview ({tts_service.engine_name}): {len(audio_bytes)} bytes")
         
         return StreamingResponse(
             iter([audio_bytes]),