[reverse-proxy] Fix gw.viktorbarzin.me — point at 192.168.1.1 via EndpointSlice

The TP-Link gateway was wired via ExternalName `gw.viktorbarzin.lan`, but
Technitium has no record for that name (the router isn't a DHCP client and
Kea DDNS never registers it), so the ingress backend returned NXDOMAIN and
the `[External] gw` Uptime Kuma monitor was permanently failing.

Factory now accepts `backend_ip` as an alternative to `external_name`: it
creates a selector-less ClusterIP Service + manual EndpointSlice pointing
at the given IP, bypassing cluster DNS entirely. Used for gw (192.168.1.1);
the old ExternalName path is retained for every other service.

Also add a direct `port` monitor for the router in uptime-kuma's
internal_monitors list so we can tell a Cloudflare/tunnel outage apart
from the router itself being down. Extended the internal-monitor-sync
script to handle non-DB monitor types (hostname + port fields).

stacks/reverse-proxy/modules/reverse_proxy/main.tf

@@ -112,13 +112,11 @@ module "idrac" {
   depends_on = [kubernetes_namespace.reverse-proxy]
 }
 
-# Can either listen on https or http; can't do both :/
-# TODO: Not working yet
 module "tp-link-gateway" {
   source             = "./factory"
   dns_type           = "proxied"
   name               = "gw"
-  external_name      = "gw.viktorbarzin.lan"
+  backend_ip         = "192.168.1.1"
   port               = 443
   tls_secret_name    = var.tls_secret_name
   backend_protocol   = "HTTPS"