Add Phase 7: grade history and session-plan item linking

- 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

packages/shared/src/schemas/index.ts

@@ -124,6 +124,8 @@ export {
   LessonPlanItemUpdateSchema,
   InstructorBlockedDateCreateSchema,
   StoreClosureCreateSchema,
+  GradeCreateSchema,
+  SessionPlanItemsSchema,
 } from './lessons.schema.js'
 export type {
   InstructorCreateInput,
@@ -145,4 +147,6 @@ export type {
   LessonPlanItemUpdateInput,
   InstructorBlockedDateCreateInput,
   StoreClosureCreateInput,
+  GradeCreateInput,
+  SessionPlanItemsInput,
 } from './lessons.schema.js'