Signals that a borrower has selected and accepted a loan offer. This endpoint is used
for applications created via the Public API where borrower communications are initially
paused.

When called, this endpoint:

• Sets the offer_selected flag to true on the application
• Records the selected product
• Sends the welcome email to the borrower
• Enables all future borrower communications (stipulation emails, etc.)
• Optionally closes any linked/competing applications

Note: This endpoint only applies to Public API-created applications. Applications
created through the Thrive UI have communications enabled by default.