Add impersonatedBy field to Session model for admin plugin

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -42,6 +42,7 @@ model Session {
   expiresAt      DateTime
   ipAddress      String?
   userAgent      String?
+  impersonatedBy String?
   createdAt      DateTime @default(now())
   updatedAt      DateTime @updatedAt