modem: add van-sms-watch to archive inbound SMS + Pushover alert

Polls ModemManager for SMS on the EC25's own flash storage, appends each
inbound message to /var/log/van-sms.jsonl, pages via Pushover (shared
van-battery creds), then deletes it from modem storage so it doesn't
silently fill up. MMS can't be decoded (WAP-push notification needs the
carrier's separate MMS APN) — detected and paged as notification-only.

Wired into deploy.sh; installed/verified live on the Pi (backlog SMS
archived + paged successfully) ahead of committing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modem/van-sms-watch.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=SMS archive + Pushover relay for the EC25 cellular modem
+After=ModemManager.service network-online.target
+Wants=ModemManager.service network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/sbin/van-sms-watch
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target