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/requirements.txt

@@ -6,3 +6,5 @@ python-multipart==0.0.6
 pydantic==2.5.3
 numpy==1.26.3
 httpx==0.26.0
+piper-tts==1.2.0
+pydub==0.25.1