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/main.py

@@ -22,7 +22,7 @@ from app.core.auth import (
 )
 from app.wave.rotator import wave_rotator
 from app.wave.playlist import ensure_smart_playlist
-from app.radio import radio_manager
+from app.radio import radio_manager, dj_host
 
 app = FastAPI(title="Navidrome Music Sync", version="1.0.0")
 
@@ -334,3 +334,16 @@ async def generate_radio_broadcast(req: Optional[RadioGenerateRequest] = None, u
     res = await radio_manager.generate_radio_broadcast(voice=voice)
     return res
 
+@app.get("/api/radio/preview-voice")
+def preview_voice(voice: str = Query(...)):
+    """Stream voice sample preview."""
+    preview_path = dj_host.get_voice_preview_path(voice)
+    if not preview_path or not os.path.exists(preview_path):
+        raise HTTPException(status_code=404, detail="Превью голоса не найдено")
+    return FileResponse(
+        preview_path,
+        media_type="audio/mpeg",
+        headers={"Content-Disposition": f"inline; filename=preview_{voice}.mp3"}
+    )
+
+