feat(radio): replace robot TTS with Piper Neural voices & add instant voice preview

- Integrate local Piper ONNX neural TTS engine (Dmitri, Irina, Denis, Ruslan)
- Add GET /api/radio/preview-voice endpoint with cached pre-rendered samples
- Add '▶ Прослушать голос' button in Web UI for 1-click preview before generating
- Add automated filter to strip LLM refusals and fall back to clean radio phrasing

app/config.py

@@ -30,7 +30,7 @@ class Settings(BaseSettings):
 
     # AI Radio DJ Settings
     RADIO_ENABLED: bool = True
-    RADIO_VOICE: str = "ru-RU-DmitryNeural"
+    RADIO_VOICE: str = "ru_RU-dmitri-medium"
     WEATHER_CITY: str = ""
     RADIO_INTRO_INTERVAL: int = 2