Purge old ledger entries.

Deletes ledger entries older than the given number of days. The ledger is kept indefinitely by default, so this is how stale entries are pruned on demand. Requires an administrator and LEDGER_DELETION_ENABLED=true. Admins may purge another user's entries by passing that user's username as owner, which additionally requires ADMIN_CROSS_USER_ACCESS_ENABLED=true.