nixos/forgejo: add age setting to automatic dump

Currently dump are added to the directory but not cleaned up.
This setting allows the user to customize the cleanup age of dumps.

doc/release-notes/rl-2511.section.md

@@ -121,6 +121,8 @@
 
 - Zig 0.12 has been removed.
 
+- `services.forgejo.dump.age` now defaults to `4w`, which deletes dumps older than 4 weeks. This new behaviour could result in older backups being deleted.
+
 - `ansible-later` has been removed because it was discontinued by the author.
 
 - `k3s` airgap images passthru attributes have changed:

nixos/modules/services/misc/forgejo.nix

@@ -285,6 +285,19 @@ in
           description = "Filename to be used for the dump. If `null` a default name is chosen by forgejo.";
           example = "forgejo-dump";
         };
+
+        age = mkOption {
+          type = types.str;
+          default = "4w";
+          example = "5d";
+          description = ''
+            Age of backup used to decide what files to delete when cleaning.
+            If a file or directory is older than the current time minus the age field, it is deleted.
+
+            The format is described in
+            {manpage}`tmpfiles.d(5)`.
+          '';
+        };
       };
 
       lfs = {
@@ -596,7 +609,7 @@ in
     };
 
     systemd.tmpfiles.rules = [
-      "d '${cfg.dump.backupDir}' 0750 ${cfg.user} ${cfg.group} - -"
+      "d '${cfg.dump.backupDir}' 0750 ${cfg.user} ${cfg.group} ${cfg.dump.age} -"
       "z '${cfg.dump.backupDir}' 0750 ${cfg.user} ${cfg.group} - -"
       "d '${cfg.repositoryRoot}' 0750 ${cfg.user} ${cfg.group} - -"
       "z '${cfg.repositoryRoot}' 0750 ${cfg.user} ${cfg.group} - -"