feat: schema — mentor assignment drop tracking

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.

prisma/migrations/20260428163421_add_mentor_drop_fields/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE "MentorAssignment" ADD COLUMN     "droppedAt" TIMESTAMP(3),
+ADD COLUMN     "droppedBy" TEXT,
+ADD COLUMN     "droppedReason" TEXT;