Add cache invalidation for the get_cached_seat_count function

[mateuszmandera]

#29258 added get_cached_seat_count which calculates and caches the number of seats being used by a Zulip Cloud organization, used for the purposes of calculating the upload storage limit (5GB/seat). Currently this cache is for 24h and nothing invalidates it, meaning an up to 24h lag between adding/removing users and the limit getting updated.

As noted in #29258 (comment) this is reasonable-ish behavior, but it's better to follow-up and add cache invalidation in the right places:

I think the callers of realm_user_count_by_role would be the right set -- so basically what you mentioned plus the role-change code path, for users switching between guests and not. Could consider changing maybe_enqueue_audit_log_upload to have a more general name like process_realm_license_count_change and do this cache flush also. Could imagine having that accept the audit log event type and do the RealmAuditLog.objects.create as well.

[zulipbot]

Hello @zulip/server-misc members, this issue was labeled with the "area: uploads" label, so you may want to check it out!