fix: use mapped table name in case_completed migration

Prisma migrate failed on prod (P3009) because DELETE targeted
"UserNotification" instead of user_notifications.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/prisma/migrations/20260907200000_case_completed_notifications/migration.sql

@@ -5,4 +5,4 @@ ALTER TYPE "LabCaseActivityType" ADD VALUE 'CASE_COMPLETED';
 ALTER TYPE "UserNotificationType" ADD VALUE 'CASE_COMPLETED';
 
 -- Step-level TASK_COMPLETED inbox rows were spam; case-complete uses CASE_COMPLETED going forward.
-DELETE FROM "UserNotification" WHERE type = 'TASK_COMPLETED';
+DELETE FROM "user_notifications" WHERE type = 'TASK_COMPLETED';