feat: integrate Silero TTS as default TTS engine

- Add SileroTTS service with GPU/CPU support
- Update TTS API to support configurable engines (silero/edge)
- Add TTS_ENGINE and SILERO_SPEAKER config options
- Set Silero as default TTS engine
- Update requirements.txt with silero dependency

.env.example

@@ -9,9 +9,15 @@ OUTPUT_DIR=./output
 SEED_VC_MODEL=seed-uvit-whisper-small-wavenet
 SEED_VC_REPO_PATH=./Seed-VC
 
+# TTS Engine Configuration
+TTS_ENGINE=silero  # Options: silero, edge
+
 # Edge-TTS Configuration
 EDGE_TTS_VOICE=ru-RU-SvetlanaNeural
 
+# Silero TTS Configuration
+SILERO_SPEAKER=v5_ru
+
 # Flask Configuration
 FLASK_PORT=5000
 FLASK_DEBUG=false