fix(phpipam): fix London SSH via WG MTU reduction (1420→1200)

Root cause: PMTU black hole on WireGuard tunnel. The tunnel runs over
the HE IPv6 6in4 tunnel (gif0 MTU 1280). With WG overhead (~80 bytes),
effective inner MTU is 1200 — but both sides were configured at 1420.
SSH kex packets >1200 bytes were silently dropped.

Fix: Set tun_wg0 MTU to 1200 on pfSense + peer_855 MTU to 1200 on
London GL-iNet. Re-enabled London DHCP/ARP import in remote CronJob.

All 3 sites now fully automated:
- Sofia: Kea leases + ARP every 5min
- London: DHCP + ARP via pfSense→London SSH hop, hourly
- Valchedrym: DHCP + ARP via pfSense→OpenWRT SSH hop, hourly

[ci skip]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

stacks/phpipam/main.tf

@@ -601,9 +601,8 @@ resource "kubernetes_cron_job_v1" "phpipam_remote_import" {
                 echo "=== Valchedrym (192.168.0.1 via pfSense) ==="
                 VALCHEDRYM=$$(ssh -o ConnectTimeout=10 admin@10.0.20.1 'timeout 15 ssh -o StrictHostKeyChecking=no -o ConnectTimeout=5 root@192.168.0.1 "cat /tmp/dhcp.leases 2>/dev/null; echo ---ARP---; cat /proc/net/arp 2>/dev/null" 2>/dev/null' 2>/dev/null || echo "")
 
-                # London: dropbear SSH kex too slow for automated use; skip for now
-                # TODO: install lightweight agent on London GL-iNet to push data
-                LONDON=""
+                echo "=== London (192.168.8.1 via pfSense) ==="
+                LONDON=$$(ssh -o ConnectTimeout=10 admin@10.0.20.1 'timeout 15 ssh -o StrictHostKeyChecking=no -o ConnectTimeout=5 root@192.168.8.1 "cat /tmp/dhcp.leases 2>/dev/null; echo ---ARP---; cat /proc/net/arp 2>/dev/null" 2>/dev/null' 2>/dev/null || echo "")
 
                 echo "=== Importing ==="
                 export LONDON_DATA="$$LONDON"