Adds 3 nullable fields to MentorAssignment for drop lifecycle:
- droppedAt: timestamp of drop (null while active)
- droppedReason: free text (required when droppedAt is set)
- droppedBy: 'mentor' | 'admin' | 'finalist_unconfirmed'
Migration is purely additive: no DROP, no ALTER COLUMN, no RENAME.
All existing rows automatically get NULL for the new columns.