feat(eientei): add replication service

spec/eientei/replication.nix

@@ -0,0 +1,43 @@
+{
+  services.syncoid = {
+    enable = true;
+    interval = "daily";
+    sshKey = "/var/lib/syncoid/.ssh/id_ed25519";
+    commonArgs = [
+      "--recursive"
+      #"--use-hold"
+      "--compress=lz4"
+      "--mbuffer-size=128M"
+    ];
+    localSourceAllow = [
+      "bookmark"
+      "hold"
+      "send"
+      "snapshot"
+      "destroy"
+      "mount"
+    ];
+
+    commands = let
+      sendOptions = "w";
+      remote = "koishi@archive:archive/backup/koishi/eientei";
+    in {
+      "eientei/persist" = {
+        inherit sendOptions;
+        target = "${remote}/persist";
+      };
+      "eientei/service" = {
+        inherit sendOptions;
+        target = "${remote}/service";
+      };
+      "eientei/storage" = {
+        inherit sendOptions;
+        target = "${remote}/storage";
+      };
+    };
+  };
+
+  users.users.syncoid.uid = 82;
+  users.groups.syncoid.gid = 82;
+  environment.persistence."/nix/persist/fhs".directories = [ "/var/lib/syncoid" ];
+}