refactor(tailscale): rename exit-node to subnet-router; drop --advertise-exit-node

The host was always intended as a LAN subnet router (--advertise-routes),
not a full exit node (--advertise-exit-node). Rename every trace of
"exit-node" to "subnet-router" and remove the --advertise-exit-node flag
from extraSetFlags; the operator supplies --advertise-routes at first
tailscale up and Tailscale persists it in state across reboots.

Routing sysctls (useRoutingFeatures = "server"), openFirewall, and
trustedInterfaces = ["tailscale0"] are still required for subnet routing
to work, so the module is kept — just correctly named.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TF2dsuKZAiyZWQ1D7CuHJm

modules/tailscale/subnet-router.nix

@@ -0,0 +1,15 @@
+_:
+
+{
+  imports = [ ./enable-service.nix ];
+
+  services.tailscale = {
+    # Enables the sysctl forwarding settings subnet routers need;
+    # without this, --advertise-routes has no effect.
+    useRoutingFeatures = "server";
+
+    # Lets peers reach this node directly over the tailscale UDP port
+    # instead of relaying through DERP.
+    openFirewall = true;
+  };
+}