sync-docs-and-fixes

docs/db/schema-changelog.md

@@ -136,10 +136,10 @@ This release extends the Classes domain to support school-level sorting and per-
 
 #### 2.2 Table: `class_subject_teachers`
 *   **Action**: `CREATE TABLE`
-*   **Primary Key**: (`class_id`, `subject`)
+*   **Primary Key**: (`class_id`, `subject_id`)
 *   **Columns**:
     *   `class_id` (varchar(128), FK -> `classes.id`, cascade delete)
-    *   `subject` (enum: `语文/数学/英语/美术/体育/科学/社会/音乐`)
+    *   `subject_id` (varchar(128), FK -> `subjects.id`, cascade delete)
     *   `teacher_id` (varchar(128), FK -> `users.id`, set null on delete)
     *   `created_at`, `updated_at`
 *   **Reason**: Maintain a stable default “subject list” per class while allowing admin/teacher to assign the actual teacher per subject.