give the staging/prod issuers a new filesystem for the voucher database

morph/lib/issuer-aws.nix

@@ -18,6 +18,17 @@
   # <https://github.com/DBCDK/morph/issues/146>.
   networking.hostName = name;
 
+  fileSystems = {
+    # Mount a dedicated filesystem (ideally on a dedicated volume, but that's
+    # beyond control of this particular part of the system) for the
+    # PaymentServer voucher database.  This makes it easier to manage for
+    # tasks like backup/recovery and encryption.
+    "voucher-database" = {
+      label = "voucher-database";
+      mountPoint = "/var/lib/zkapissuer-vouchers";
+    };
+  };
+
   # Clean up packages after a while
   nix.gc = {
     automatic = true;