Fix notify_to env var name (fallback)

backend/api.py

@@ -7,6 +7,15 @@ from typing import Any, Dict
 
 from dotenv import load_dotenv
 from fastapi import FastAPI, HTTPException, UploadFile, File, Form
+
+def _env(*keys: str, default: str = "") -> str:
+    import os
+    for k in keys:
+        v = (os.environ.get(k) or "").strip()
+        if v:
+            return v
+    return default
+
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse, PlainTextResponse