v73: Version 73 (July 11, 2025)

Mail:

* Quotas for mail storage can now be set per user in the control panel.
* Autoconfig now includes POP3 and CardDAV/CalDAV.

Backups:

* Fix for S3-compatible backups (other than AWS S3 itself).

Control Panel:

* Backup status is added to the status checks.
* S3 backup credentials can now be stored in environment variables.
* Fix for when an AAAA record is set up the box's own IP address.
* Fix for when logged out of the control panel.
* Fix link to Z-Push client compatibility list.

Setup:

* The Ubuntu version check is updated.

Other:

* Code cleanup using the Ruff Python linter.
* Other minor changes.