Clear error when HF_TOKEN secret is missing

app.py

@@ -45,6 +45,10 @@ GREEN = np.array([120, 255, 155], np.float32)
 # ---------------------------------------------------------------- models
 print("[startup] downloading VideoMaMa++ (private repo)...", flush=True)
 _local = os.environ.get("VIDEOMAMAPP_LOCAL_DIR")  # local testing only
+if not _local and not HF_TOKEN:
+    raise RuntimeError(
+        "HF_TOKEN is not set. Add a read token with access to the private model repo "
+        f"{PRIVATE_REPO} and to facebook/sam3 as a Space secret named HF_TOKEN.")
 _private = Path(_local) if _local else Path(snapshot_download(PRIVATE_REPO, token=HF_TOKEN))
 sys.path.insert(0, str(_private / "code"))
 import videomamapp_inference as vmpp  # noqa: E402