ahmed: Factor configuration.nix into smaller submodules

I checked that this didn't result in any (non-trivial) changes like so:

$ vi hosts/ahmed/configuration.nix
$ nixos-rebuild build --flake .#ahmed
$ nix-diff /run/current-system ./result

1 files changed, 20 insertions, 0 deletions

diff --git a/hosts/ahmed/dyndns/default.nix b/hosts/ahmed/dyndns/default.nix
new file mode 100644
index 0000000..62c2cca
--- /dev/null
+++ b/hosts/ahmed/dyndns/default.nix
@@ -0,0 +1,20 @@
+# This module sets up dynamic DNS (DDNS).
+#
+# Other services will register the domains to be updated via
+# `services.cloudflare-dyndns.domains`.
+{config, ...}: {
+  services.cloudflare-dyndns = {
+    enable = true;
+    apiTokenFile = config.age.secrets.cloudflare-dyndns-api-token.path;
+    proxied = true;
+  };
+
+  # We have to authenticate the Cloudflare's DDNS service with an API key.
+  age.secrets.cloudflare-dyndns-api-token.file = ../../../secrets/cloudflare-ddns-token.env.age;
+
+  # We also have to overwrite the dependencies of the DYNDNS client service to
+  # make sure we are *actually* online.
+  #
+  # See: https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget
+  systemd.services.cloudflare-dyndns.after = ["network-online.target"];
+}