fix: add scene preview image route

app.py

@@ -96,6 +96,15 @@ async def dashboard_preview(theme_id: str, ext: str):
     raise HTTPException(404)
 
 
+@app.get("/scene/{scene_id}/preview.{ext}")
+async def scene_preview(scene_id: str, ext: str):
+    from models.scene import SCENES_DIR
+    path = SCENES_DIR / scene_id / f"preview.{ext}"
+    if path.exists():
+        return FileResponse(path)
+    raise HTTPException(404)
+
+
 @app.get("/scene/{scene_id}", response_class=HTMLResponse)
 async def scene_render(request: Request, scene_id: str):
     from models.scene import scene_manager