ap: keep AP beaconing across USB re-enumeration

Starlink's USB ethernet shares a hub with the rtw89 Wi-Fi adapter; when Starlink
flaps, the hub re-enumerates and tears the radio down, so hostapd loses its
interface and exits. Two stock defaults left the AP dark: Restart=on-failure
misses clean exits, and the 5-in-10s start limit makes systemd give up during a
re-enumeration storm.

- hostapd-restart.conf drop-in: Restart=always, RestartSec=5, StartLimitIntervalSec=0
  so hostapd retries forever until the interface returns (verified: SIGKILL -> back
  to state=ENABLED in ~7s).
- van-ap-watchdog daemon: backstop for the case systemd can't see (hostapd running
  but radio wedged). Polls hostapd_cli status; restarts hostapd if the iface is
  present but not ENABLED, waits it out if the iface is mid-re-enumeration.
- hostapd.conf: add ctrl_interface so the watchdog can read the real AP state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ap/hostapd.conf

@@ -3,6 +3,10 @@ interface=wlxc83a35a4ee55
 # hostapd adds the wlan to the bridge after setting AP mode; the bridge itself + its
 # wired member + the gateway IP are defined under /etc/systemd/network (2x-van-br0/lan).
 bridge=br0
+# Control socket so van-ap-watchdog (and hostapd_cli) can read the AP's real state
+# (state=ENABLED means it's actually beaconing) — used to detect a wedged radio that
+# stays "running" but stops serving after a USB glitch.
+ctrl_interface=/var/run/hostapd
 driver=nl80211
 ssid=VanLink
 country_code=CA