caddy: wildcard TLS via DNS-01 challenge + ddns-updater for Njalla

Build Caddy with the caddy-dns/njalla plugin to enable DNS-01 ACME
challenges. This issues a single wildcard certificate for
*.sigkill.computer instead of per-subdomain certificates, reducing
Let's Encrypt API calls and certificate management overhead.

Add ddns-updater service (nixpkgs services.ddns-updater) configured
with Njalla provider to automatically update DNS records when the
server's public IP changes.

services/ddns-updater.nix

@@ -0,0 +1,14 @@
+{
+  config,
+  ...
+}:
+{
+  services.ddns-updater = {
+    enable = true;
+    environment = {
+      PERIOD = "5m";
+      # ddns-updater reads config from this path at runtime
+      CONFIG_FILEPATH = config.age.secrets.ddns-updater-config.path;
+    };
+  };
+}