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-heartbeat

@@ -37,12 +37,9 @@ PAYLOAD="{\"mac\":\"${MAC}\",\"hostname\":\"${HOSTNAME}\",\"yggdrasil_address\":
 
 RESPONSE=""
 
-# Use longer timeout if called from vpn-tunnel init (first boot)
-CURL_TIMEOUT="${HEARTBEAT_CURL_TIMEOUT:-10}"
-
 if [ "$ROLE" = "bee" ]; then
     # Bee: no yggdrasil, use public URL only
-    RESPONSE=$(curl -s -m "$CURL_TIMEOUT" -X POST \
+    RESPONSE=$(curl -s -m 10 -X POST \
         -H "Content-Type: application/json" \
         -H "Authorization: Bearer ${HEARTBEAT_KEY}" \
         -d "$PAYLOAD" \
@@ -50,13 +47,13 @@ if [ "$ROLE" = "bee" ]; then
 else
     # Bumblebee: try yggdrasil first, fallback to public
     if ping6 -c 1 -W 3 200:abb9:5810:37d3:8a4c:98a6:b82b:969a >/dev/null 2>&1; then
-        RESPONSE=$(curl -s -m "$CURL_TIMEOUT" -X POST \
+        RESPONSE=$(curl -s -m 10 -X POST \
             -H "Content-Type: application/json" \
             -H "Authorization: Bearer ${HEARTBEAT_KEY}" \
             -d "$PAYLOAD" \
             "${PARAHUB_API_YGG}" 2>/dev/null)
     else
-        RESPONSE=$(curl -s -m "$CURL_TIMEOUT" -X POST \
+        RESPONSE=$(curl -s -m 10 -X POST \
             -H "Content-Type: application/json" \
             -H "Authorization: Bearer ${HEARTBEAT_KEY}" \
             -d "$PAYLOAD" \
@@ -64,22 +61,6 @@ else
     fi
 fi
 
-# Parse tunnel_ip from response and update local config (Bumblebee only)
-if [ "$ROLE" != "bee" ] && [ -n "$RESPONSE" ]; then
-    NEW_TUNNEL_IP=$(echo "$RESPONSE" | jsonfilter -e '$.tunnel_ip' 2>/dev/null)
-    if [ -n "$NEW_TUNNEL_IP" ]; then
-        OLD_TUNNEL_IP=$(cat /etc/parahub/tunnel_ip 2>/dev/null)
-        if [ "$OLD_TUNNEL_IP" != "$NEW_TUNNEL_IP" ]; then
-            echo "$NEW_TUNNEL_IP" > /etc/parahub/tunnel_ip
-            logger -t parahub-heartbeat "Tunnel IP updated: ${OLD_TUNNEL_IP:-none} → $NEW_TUNNEL_IP"
-            # Restart vpn-tunnel if it's running (IP changed)
-            if [ -n "$OLD_TUNNEL_IP" ]; then
-                /etc/init.d/parahub-vpn-tunnel restart 2>/dev/null &
-            fi
-        fi
-    fi
-fi
-
 # Sync paid_clients to speed control (Bumblebee only)
 if [ "$ROLE" != "bee" ] && [ -x /usr/bin/parahub-speed-control ] && [ -n "$RESPONSE" ]; then
     PAID_IPS=$(echo "$RESPONSE" | jsonfilter -e '$.paid_clients[*]' 2>/dev/null)