refactor: Remove GRE tunnels, add Mullvad gateway health check

Radical simplification — no more VPS data plane:
- Delete parahub-vpn-tunnel init script (GRE6 no longer used)
- Revert heartbeat to clean version (no tunnel_ip parsing)
- Add parahub-gw-check: monitors WireGuard handshake, switches
  batman-adv gw_mode between server/client (cron every 2 min)
- Update uci-defaults: remove vpn_tunnel zone/interface, start
  bumblebee as gw_mode=client (health check promotes to server)

Guest internet now requires Mullvad — kill switch by design.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

files/usr/bin/parahub-gw-check

@@ -0,0 +1,36 @@
+#!/bin/sh
+# Parahub Gateway Health Check
+# Monitors Mullvad WireGuard status and switches batman-adv gw_mode:
+#   - Mullvad active (recent handshake) → gw_mode=server (advertise as gateway)
+#   - Mullvad down or not configured    → gw_mode=client (relay only)
+#
+# Run via cron every 2 minutes. Bumblebee only.
+
+[ "$(cat /etc/parahub/role 2>/dev/null)" = "bumblebee" ] || exit 0
+
+# Check if Mullvad WireGuard interface exists and has recent handshake
+MULLVAD_OK=0
+if command -v wg >/dev/null 2>&1; then
+    # Get latest handshake timestamp from any WireGuard interface
+    LAST_HS=$(wg show all latest-handshakes 2>/dev/null | awk '{print $NF}' | sort -rn | head -1)
+    if [ -n "$LAST_HS" ] && [ "$LAST_HS" -gt 0 ] 2>/dev/null; then
+        NOW=$(date +%s)
+        AGE=$((NOW - LAST_HS))
+        # Handshake within last 5 minutes = alive
+        [ "$AGE" -lt 300 ] && MULLVAD_OK=1
+    fi
+fi
+
+CURRENT_MODE=$(batctl gw_mode 2>/dev/null | awk '{print $1}')
+
+if [ "$MULLVAD_OK" = "1" ]; then
+    if [ "$CURRENT_MODE" != "server" ]; then
+        batctl gw_mode server
+        logger -t parahub-gw "Mullvad active, switched to gw_mode=server"
+    fi
+else
+    if [ "$CURRENT_MODE" != "client" ]; then
+        batctl gw_mode client
+        logger -t parahub-gw "Mullvad down, switched to gw_mode=client"
+    fi
+fi