ruan: replace subvolume mount with impermanence

hosts/ruan/default.nix

···
           "/var/lib/private/radicale"
         ];
       };
+      environment.persistence."/media/data/persist" = {
+        hideMounts = true;
+        directories = [ "/var/lib/transmission" ];
+      };
     };
 }

hosts/ruan/hardware-configuration.nix

···
       ];
     };
 
-    fileSystems."/var/lib/transmission" = {
-      device = "/dev/disk/by-uuid/367ffdb7-bfaf-4409-9115-5ecbe4261bae";
-      fsType = "btrfs";
-      options = [
-        "subvol=transmission"
-        "defaults"
-        "compress=zstd"
-      ];
-    };
-
     fileSystems."/media/leiston" = {
       device = "/dev/disk/by-uuid/c76abfad-5b5a-478f-a85e-dde18ffe202f";
       fsType = "btrfs";