feat: Add Piper TTS integration with pitch control support

- Created PiperTTSService with pitch shifting (-12 to +12 semitones)
- Created UnifiedTTSService to manage multiple TTS engines
- Added engine selection endpoints (GET/POST /api/tts/engines)
- Added voices listing endpoint (GET /api/tts/voices)
- Added pitch_adjustments parameter to TTS generation
- Created install_piper.sh for easy Piper installation
- Updated backend README with multi-engine documentation
- Created comprehensive PIPER_TTS_GUIDE.md
- Added TTS_ENGINE environment variable for engine selection
- Updated requirements.txt with piper-tts and pydub

This allows users to choose between Kokoro (high quality) and Piper
(fast with pitch control) TTS engines at runtime.

backend/.env.example

@@ -1,3 +1,4 @@
 PORT=8000
 LOG_LEVEL=info
 OLLAMA_BASE_URL=http://localhost:11434
+TTS_ENGINE=kokoro