Applicant Verifier Select field designation

Problem

A Formnflow form template can contain multiple Verifier Select fields — for example, one where the applicant chooses their verifier and a separate one where staff record a verifier for an individual team member. Previously, every Verifier Select field triggered assign_verifier! when its value changed, so any field could silently overwrite the funding application's verifier.

How it works

A new boolean column, primary_verifier_select, is added to each Verifier Select field. Only the field flagged as the primary verifier select drives assign_verifier!; all others store their value normally without side effects.

Default behaviour on creation:

• The first Verifier Select field added to a template is automatically designated as the primary one (primary_verifier_select: true).

• Every subsequent Verifier Select field defaults to false.

Mutual exclusion:

• Enabling primary_verifier_select on one field automatically disables it on all other Verifier Select fields in the same template, ensuring exactly one field controls verifier assignment at any time.

UI changes

In the form builder, a checkbox labelled "Applicant verifier select" appears in the field context bar whenever a Verifier Select field is focused. Checking it designates that field as the primary verifier select; unchecking designates it as a secondary field.

Data migration

Existing templates require a one-off data migration (see DATA_MIGRATIONS.md) to mark the earliest Verifier Select field per template as the primary one.