- New tables: lesson_plan_item_grade_history (append-only), lesson_session_plan_item
- Grading an item updates current_grade_value and creates immutable history record
- Grading a not_started item auto-transitions it to in_progress
- Linking items to a session also auto-transitions not_started items
- Link operation is idempotent — re-linking same items produces no duplicates
- Endpoints: POST/GET /lesson-plan-items/:id/grades, GET /lesson-plan-items/:id/grade-history
- Endpoints: POST/GET /lesson-sessions/:id/plan-items
- 8 new integration tests
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