Custom form fields: select options + required validation + file field

[RikepilB]

Problem

Custom form fields are a P0 gap (per docs/CLAUDE.md). The form builder UI exists but:

• dropdown/select fields don't persist or render their options
• required validation isn't enforced on submit
• custom file field behavior is unspecified/untested

Fix

• Persist select options with the field definition; render them on the public apply form.
• Enforce required server-side (Zod) and client-side on the public form.
• Define + test custom file-field upload (size/type limits, storage path).
• Round-trip: build field → appears on careers form → submitted value saved on the application.

Acceptance

• Select field with options persists + renders + saves chosen value
• Required custom field blocks submit when empty (client + server)
• Custom file field uploads + is retrievable
• Tests for schema validation + persistence

Notes

Migration for customFields column already added in an earlier session. Check whether persistence was wired.