Add Long Text to custom field type selector

[rsaether76]

Summary

Adds Paperless-ngx longtext as a selectable custom field type in the Paperless-AI Next settings UI.

Paperless-AI Next already preserves configured longtext custom fields through CUSTOM_FIELDS, and the shared custom field validator only applies the 128-character length guard to string fields. This change lets users select long text fields from the settings page instead of manually editing Docker environment JSON.

Changes

• Adds Long Text / longtext to the custom field type dropdown in views/settings.ejs.
• Adds a lightweight regression test that checks the settings template exposes the longtext option.
• Registers the new test in the existing test runner under the processing area.

Testing

node scripts/run-tests.js --test custom-field-longtext-ui
node --check scripts/run-tests.js
git diff --check