Added #15312: Add checkin due in days setting

[Glukose1]

This PR implements a setting to specify the number of days before an asset appears on the due for check-in page. Currently, this uses the "audit warning days" setting, but a separate setting allows for different values for each field.

Fixes #15312

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Different values have been assigned for both settings and both are saved correctly. The "Due for Checkin" page references the new setting.

Test Configuration:

• PHP version: 8.2.20
• MySQL version: Ver 15.1 Distrib 10.11.6-MariaDB
• Webserver version: Apache/2.4.59 (Debian)
• OS version: Debian GNU/Linux 12.6 (bookworm)

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[what-the-diff]

PR Summary

• Added 'due_checkin_days' property to Settings Controller
  This enhancement allows the application to handle properties related to the time frame for due check-ins, defined within our system.

• Modified interval variable to use 'due_checkin_days' property
  This change allows for better integration of the 'due_checkin_days' property within our assets model, making our time tracking components more efficient when calculating due dates.

• Added language resource entries for 'due_checkin_days'
  To improve our international user experience, we have integrated new language translation entries for 'due_checkin_days' in our English language resource file.

• Updated title on hardware check-in page
  The page title on the hardware check-in has been updated to reflect the new 'due_checkin_days' property, ultimately giving our users better context.

• Integration of form field for 'due_checkin_days' in settings
  A new form field has been introduced in the alert settings page concerning 'due_checkin_days'. This field increases ease of use for our clients and boosts self-service capabilities on the platform.

[snipe]

Thanks for this - this looks good. Trying to figure out why the tests are failing.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[snipe]

Uh, wait, aren't we missing a migration here?

[Glukose1]

Indeed, idk why the migration wasn't included in the commit.

[snipe]

It's all good, I added it