[eric] runner: per-app VM image boots a published bundle from storage and serves backend + frontend from one process (ENG-293)

openswarm-apprunner/fallback.py

@@ -0,0 +1,11 @@
+"""Honest degraded mode: the bundle could not be fetched (or has no backend); every request says
+so instead of crash-looping the VM."""
+from fastapi import FastAPI
+from fastapi.responses import JSONResponse
+
+app = FastAPI()
+
+
+@app.get("/{path:path}")
+async def unavailable(path: str) -> JSONResponse:
+    return JSONResponse({"error": "this app's backend is not available right now"}, status_code=503)