feat(cosmos): add simple ping watchdog

Signed-off-by: Sefa Eyeoglu <[e-mail]>

nixosConfigurations/cosmos/configuration.nix

···
     ./traefik.nix
     ./wifi.nix
     ./wireguard.nix
+    ./watchdog.nix
 
     fpConfig.flake.modules.nixos.raspberry-pi-4
     fpConfig.flake.modules.nixos.ext-docker

nixosConfigurations/cosmos/watchdog.nix

···
+{pkgs, ...}: {
+  systemd.services."ping-watchdog" = {
+    description = "Watchdog checking for network access";
+    onFailure = ["reboot.target"];
+    path = [pkgs.iputils];
+
+    script = ''
+      ping -c 4 -W 1 10.0.0.1
+    '';
+  };
+
+  systemd.timers."ping-watchdog" = {
+    after = ["network.target"];
+    wantedBy = ["timers.target"];
+    timerConfig = {
+      OnCalendar = "minutely";
+      OnActiveSec = "2m";
+    };
+  };
+}