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/README.md

@@ -83,6 +83,15 @@ Or install manually:
 piper --version
 ```
 
+**Download Voice Models:**
+
+Piper requires manual voice model download. See [PIPER_SETUP.md](PIPER_SETUP.md) for detailed instructions.
+
+Quick steps:
+1. Visit https://github.com/rhasspy/piper/releases/tag/v1.2.0
+2. Download voice models (e.g., `en_US-lessac-medium.onnx` and `.onnx.json`)
+3. Place in `backend/piper_models/` directory
+
 **Benefits of Piper TTS:**
 - ✨ Pitch shifting support (-12 to +12 semitones per role)
 - ✨ Fast, lightweight neural TTS