add a prune timer to the backup server

modules/restic-server/default.nix

@@ -17,4 +17,25 @@ in
     extraFlags = [ "--no-auth" ];
   };
   networking.firewall.allowedTCPPorts = [ 8000 ];
+
+  systemd.services.prune-restic = {
+    serviceConfig.Type = "oneshot";
+    script = ''
+      ${pkgs.restic}/bin/restic \
+      --repo ${repository} \
+      --password-file "/home/${custom.username}/.nixos/secrets/passwords/restic.key" \
+      forget \
+        --keep-daily 7 \
+        --keep-weekly 5 \
+        --keep-monthly 12 \
+        --keep-yearly 75 \
+        --prune
+    '';
+  };
+
+  systemd.timers.prune-restic = {
+    wantedBy = [ "timers.target" ];
+    partOf = [ "prune-restic.service" ];
+    timerConfig.OnCalendar = [ "*-*-* 12:00:00" ];
+  };
 }