feat: add AUR update delay setting

[wandercone]

This patch adds a new setting, AURUpdateDelayDays, that delays showing AUR updates until the upstream package has been published for at least the configured number of days. It is intended as a lightweight age gate that gives the community time to flag malicious or broken AUR uploads before they are automatically offered to regular users.

Users accept very real risks when installing software from the AUR. As one forum moderator put it: "...one can explain that the soup is hot, but that does not prevent those hungry from burning their lips.". This change gives the user a thermometer, an optional delay that lets AUR packages cool down before they appear in the update scan.

Workflow:

• The delay is measured in whole days (0 = disabled, default).
• The age is taken from the AUR RPC LastModified timestamp.
• The filter is applied only during the normal update scan (get_updates_real).
• It is not applied when the transaction layer explicitly collects AUR packages to build (get_aur_updates_async), so manual installs and full upgrades can still build any selected package.