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/schema.prisma

@@ -1285,6 +1285,11 @@ model MentorAssignment {
   completionStatus String    @default("in_progress") // 'in_progress' | 'completed' | 'paused'
   lastViewedAt     DateTime?
 
+  // Drop tracking — null while assignment is active
+  droppedAt     DateTime?
+  droppedReason String?   @db.Text
+  droppedBy     String? // 'mentor' | 'admin' | 'finalist_unconfirmed'
+
   // ── Competition/Round architecture — workspace activation ──
   workspaceEnabled Boolean   @default(false)
   workspaceOpenAt  DateTime?