hotfix: re-add hypercorn with railway start cmd/logging.toml, restore internal uvicorn bindings to conditional ip-v4/v6

backend/railway.json

@@ -4,6 +4,6 @@
     "builder": "NIXPACKS"
   },
   "deploy": {
-    "startCommand": "python3 -m linkpulse serve"
+    "startCommand": "hypercorn linkpulse.app:app --bind \"[::]:$PORT\" --log-config \"toml:logging.toml\""
   }
-}
+}