Make legacy migrations safe on canonical schema

backend/migrations/20260908_runtime_schema_consolidation.sql

@@ -1,13 +1,6 @@
 -- Consolidates additive schema previously created lazily by request handlers.
 -- Run once on staging/production before disabling runtime DDL.
 
-ALTER TABLE `teachers`
-  ADD COLUMN `grades_taught` TEXT NULL,
-  ADD COLUMN `school_name` VARCHAR(255) NULL;
-
-ALTER TABLE `exam_attempts`
-  ADD COLUMN `completed_at` TIMESTAMP NULL DEFAULT NULL AFTER `ai_diagnostic_report`;
-
 CREATE TABLE IF NOT EXISTS `teacher_reviews` (
   `id` BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
   `teacher_id` BIGINT UNSIGNED NOT NULL,
@@ -31,4 +24,3 @@ CREATE TABLE IF NOT EXISTS `teacher_reviews` (
   CONSTRAINT `fk_tr_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teachers` (`id`) ON DELETE CASCADE,
   CONSTRAINT `fk_tr_student` FOREIGN KEY (`student_id`) REFERENCES `students` (`id`) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
-