Back up Knot DNSSEC keystore from ns1 to control via borgbackup

clan borgbackup instance: control serves repos, ns1 backs up its
clan.core.state (the KASP keystore at /var/lib/knot) nightly over the
mesh with repokey encryption. ns1 maps the control machine name to its
ZeroTier address so the borg@control repo resolves.

Run `clan vars generate ns1` before deploy to mint the borg keypair.

clan.nix

@@ -64,6 +64,16 @@ in
     emergency-access = {
       roles.default.tags.nixos = { };
     };
+
+    # Encrypted, deduplicating backups. control hosts the repos; ns1 is the
+    # only client, backing up its declared clan.core.state (the Knot DNSSEC
+    # keystore) over the mesh. Repo lives at /var/lib/borgbackup/ns1 on control.
+    # Cross-host so an ns1 loss is recoverable; repokey encryption means control
+    # never holds plaintext. Run `clan vars generate ns1` (YubiKey) before deploy.
+    borgbackup = {
+      roles.server.machines.control = { };
+      roles.client.machines.ns1 = { };
+    };
   };
 
   machines = {