Add an option to provide a description on a field.

[jtphelan]

It would be nice to be able to set a description or help text that displays below a custom field.

[ManukMinasyan]

Implemented in #94.

This will be available in the next release. The implementation:

• Uses the existing settings JSON column (no migration needed)
• Adds a description property to the settings DTO
• Renders as helper text below fields in end-user forms
• Gated behind a FIELD_DESCRIPTION feature flag (enabled by default)