Add AI% field to applications

[balebbae]

Description

Add an AI percentage field to the application model. Admins can edit the field on their assigned applications only if it hasn't been set yet — once saved, it becomes read-only.

Requirements

• Add an ai_percentage field to the application model and database schema
• Create a migration for the new column
• Admins can set the AI% field on applications assigned to them
• The field is editable only if it hasn't been set yet (null/empty)
• Once saved, the field becomes read-only and cannot be modified
• Display the AI% field in the admin application review UI

Acceptance Criteria

• Database migration adds ai_percentage field to applications table
• Backend supports updating AI% only if not already set and only by the assigned admin
• Frontend displays AI% field in admin review view
• Field is editable when unset, read-only once saved
• Appropriate validation and error handling