feat: VPS WireGuard gateway auto-configuration (ph4)

- uci-defaults: WG keygen, vps_gateway interface+zone+forwarding
- heartbeat: sends wg_public_key, parses VPS config, calls vps-setup
- parahub-vps-setup: new script for auto-configuring VPS tunnel with
  OTA bootstrap support and idempotent state tracking
- parahub-mullvad: setup disables vps_gateway, remove re-enables it
  (fixes bug referencing non-existent vpn_tunnel interface)
- parahub-gw-check: works with both vps_gateway and mullvad_local
- sysupgrade.conf: preserves WG VPS keys across upgrades
- build.sh: bump PARAHUB_BUILD to 4

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

scripts/build.sh

@@ -11,7 +11,7 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 
 OPENWRT_VERSION="${OPENWRT_VERSION:-25.12.0-rc4}"
-PARAHUB_BUILD="3"
+PARAHUB_BUILD="4"
 FIRMWARE_VERSION="${OPENWRT_VERSION}-ph${PARAHUB_BUILD}"
 
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