refactor: streamline core modules into dedicated folder

modules/core/fail2ban.nix

@@ -0,0 +1,27 @@
+{ ... }:
+{
+  services.fail2ban = {
+    enable = true;
+    ignoreIP = [
+      "141.30.0.0/16"
+      "141.76.0.0/16"
+    ];
+    bantime-increment = {
+      enable = true;
+    };
+
+    jails = {
+      dovecot = ''
+        enabled = true
+        # aggressive mode to add blocking for aborted connections
+        filter = dovecot[mode=aggressive]
+        maxretry = 3
+      '';
+      postfix = ''
+        enabled = true
+        filter = postfix[mode=aggressive]
+        maxretry = 3
+      '';
+    };
+  };
+}