Locks an application for editing and prepares it for credit qualification.

Business Context:
This endpoint finalizes the application data and moves it into the qualification
pipeline. After submission, the application cannot be edited without reopening it first.

Prerequisites:

• Application must be in open status
• All required fields must be completed and valid
• Customer consents must be obtained (consent1, consent2, consent3)

Status Transition:
open → submitted

After Submission:

• Application becomes read-only
• Ready for qualification process
• Must use /applications/{applicationId}/qualify endpoint to start credit checks
• Can be reopened if changes are needed