failover: fix Starlink interface + template config.json from deploy.conf

STARLINK_IFACE pointed at the wrong device after the USB hub swap (was
accidentally set to the 5GHz AP dongle's interface). failover/config.json
also hardcoded the old interface name instead of being templated like the
other configs, so deploy.conf changes never reached it — switched it to
@STARLINK_IFACE@ and deploy.sh now renders it.

failover/config.json

@@ -10,7 +10,7 @@
   ],
   "wans": [
     { "name": "wifi",     "device": "wlan0",           "metric": 100 },
-    { "name": "starlink", "device": "enxd8ec5eeb3512", "metric": 200 },
+    { "name": "starlink", "device": "@STARLINK_IFACE@", "metric": 200 },
     { "name": "cellular", "connection": "Koodo",       "metric": 300 }
   ]
 }