fix: use original index name in security_event drop migration down()

Match the index name IDX_security_event_user_id_timestamp from the
AddPerformanceIndexes migration to avoid schema drift on rollback.

Author: brunobuddy

packages/backend/src/database/migrations/1773700000000-DropSecurityEventTable.ts

@@ -18,7 +18,7 @@ export class DropSecurityEventTable1773700000000 implements MigrationInterface {
       )
     `);
     await queryRunner.query(
-      `CREATE INDEX "IDX_security_event_user_timestamp" ON "security_event" ("user_id", "timestamp")`,
+      `CREATE INDEX "IDX_security_event_user_id_timestamp" ON "security_event" ("user_id", "timestamp")`,
     );
   }
 }