add aspects/hosts/ configurations

Host-specific NixOS configurations for:
- alexandria (server)
- io (desktop)
- rotterdam (desktop)
- trantor (server, aarch64)

Each host has a main config file and _hostname/ directory
with hardware-configuration and other host-specific modules.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

aspects/hosts/_trantor/openssh.nix

@@ -0,0 +1,23 @@
+{ ... }:
+
+{
+  services = {
+    openssh = {
+      settings = {
+        PasswordAuthentication = false;
+        KbdInteractiveAuthentication = false;
+      };
+    };
+    fail2ban.jails.sshd = {
+      settings = {
+        enabled = true;
+        port = "ssh";
+        filter = "sshd";
+        logpath = "/var/log/auth.log";
+        maxretry = 3;
+        findtime = "10m";
+        bantime = "1h";
+      };
+    };
+  };
+}