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/test_piper_direct.py

@@ -0,0 +1,94 @@
+"""
+Direct test of Piper TTS functionality.
+"""
+import subprocess
+import os
+
+def test_piper_installation():
+    """Test if Piper is installed and accessible."""
+    print("Testing Piper Installation\n" + "="*50)
+    
+    # Check if piper command exists
+    try:
+        result = subprocess.run(
+            ["piper", "--version"],
+            capture_output=True,
+            text=True,
+            timeout=5
+        )
+        print(f"✓ Piper found: {result.stdout.strip()}")
+        return True
+    except FileNotFoundError:
+        print("✗ Piper not found in PATH")
+        print("  Install with: ./install_piper.sh")
+        return False
+    except Exception as e:
+        print(f"✗ Error checking Piper: {e}")
+        return False
+
+
+def test_piper_synthesis():
+    """Test basic Piper synthesis."""
+    print("\nTesting Piper Synthesis\n" + "="*50)
+    
+    # Create models directory
+    models_dir = os.path.join(os.path.dirname(__file__), "piper_models")
+    os.makedirs(models_dir, exist_ok=True)
+    print(f"Models directory: {models_dir}")
+    
+    # Try to synthesize with default voice
+    voice = "en_US-lessac-medium"
+    model_path = os.path.join(models_dir, f"{voice}.onnx")
+    
+    if not os.path.exists(model_path):
+        print(f"\n⚠️  Model not found: {model_path}")
+        print(f"Attempting to download {voice}...")
+        
+        # Try downloading
+        try:
+            import urllib.request
+            base_url = "https://huggingface.co/rhasspy/piper-voices/resolve/v1.0.0"
+            model_url = f"{base_url}/en_US/{voice}.onnx"
+            config_url = f"{base_url}/en_US/{voice}.onnx.json"
+            
+            print(f"Downloading from: {model_url}")
+            urllib.request.urlretrieve(model_url, model_path)
+            urllib.request.urlretrieve(config_url, f"{model_path}.json")
+            print("✓ Download complete")
+        except Exception as e:
+            print(f"✗ Download failed: {e}")
+            return False
+    else:
+        print(f"✓ Model found: {model_path}")
+    
+    # Try synthesis
+    try:
+        print("\nAttempting synthesis...")
+        process = subprocess.Popen(
+            ["piper", "--model", model_path, "--output-raw"],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE
+        )
+        
+        test_text = "Hello, this is a test."
+        stdout, stderr = process.communicate(input=test_text.encode('utf-8'), timeout=10)
+        
+        if process.returncode == 0:
+            print(f"✓ Synthesis successful: {len(stdout)} bytes of audio")
+            return True
+        else:
+            print(f"✗ Synthesis failed (exit code {process.returncode})")
+            print(f"  Error: {stderr.decode()}")
+            return False
+            
+    except Exception as e:
+        print(f"✗ Synthesis error: {e}")
+        return False
+
+
+if __name__ == "__main__":
+    if test_piper_installation():
+        test_piper_synthesis()
+    
+    print("\n" + "="*50)