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/PIPER_SETUP.md

@@ -0,0 +1,98 @@
+# Piper TTS Setup Guide
+
+## Quick Start
+
+Piper TTS is now integrated but requires manual voice model setup.
+
+### 1. Install Piper
+
+```bash
+cd backend
+./install_piper.sh
+```
+
+Verify installation:
+```bash
+piper --version
+```
+
+### 2. Download Voice Models
+
+Piper voice models need to be downloaded manually and placed in `backend/piper_models/`.
+
+**Download voices from:**
+https://github.com/rhasspy/piper/releases/tag/v1.2.0
+
+**Recommended voices for English:**
+
+1. **en_US-lessac-medium** (Male, American)
+   - Download: `en_US-lessac-medium.onnx` and `en_US-lessac-medium.onnx.json`
+   - Place in: `backend/piper_models/`
+
+2. **en_US-amy-medium** (Female, American)
+   - Download: `en_US-amy-medium.onnx` and `en_US-amy-medium.onnx.json`
+   - Place in: `backend/piper_models/`
+
+3. **en_GB-alan-medium** (Male, British)
+   - Download: `en_GB-alan-medium.onnx` and `en_GB-alan-medium.onnx.json`
+   - Place in: `backend/piper_models/`
+
+### 3. Directory Structure
+
+After downloading, your directory should look like:
+
+```
+backend/
+├── piper_models/
+│   ├── en_US-lessac-medium.onnx
+│   ├── en_US-lessac-medium.onnx.json
+│   ├── en_US-amy-medium.onnx
+│   ├── en_US-amy-medium.onnx.json
+│   ├── en_GB-alan-medium.onnx
+│   └── en_GB-alan-medium.onnx.json
+└── ...
+```
+
+### 4. Test Piper
+
+```bash
+cd backend
+python test_piper_direct.py
+```
+
+### 5. Use in Application
+
+1. Start the backend server
+2. In the frontend, click "Piper TTS" button in Voice Assignment screen
+3. Select Piper voices from the dropdown
+4. Generate audiobook with pitch control!
+
+## Troubleshooting
+
+### "Model not found" error
+
+- Make sure you downloaded both `.onnx` and `.onnx.json` files
+- Check files are in `backend/piper_models/` directory
+- Verify filenames match exactly (case-sensitive)
+
+### "Piper not found" error
+
+- Run `./install_piper.sh` to install Piper
+- Or install manually from https://github.com/rhasspy/piper/releases
+
+### Synthesis fails
+
+- Check Piper version: `piper --version`
+- Try running Piper directly: `echo "test" | piper --model piper_models/en_US-lessac-medium.onnx --output-raw > test.wav`
+- Check backend logs for detailed error messages
+
+## Alternative: Use Kokoro TTS
+
+If Piper setup is too complex, you can continue using Kokoro TTS (the default engine) which works out of the box with no additional setup.
+
+## More Voices
+
+Browse all available Piper voices:
+https://rhasspy.github.io/piper-samples/
+
+Download any voice and place the `.onnx` and `.onnx.json` files in `backend/piper_models/`.