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/backend/src/db/schema/lessons.ts

@@ -218,6 +218,30 @@ export const lessonPlanItems = pgTable('lesson_plan_item', {
   updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
 })
 
+export const lessonPlanItemGradeHistory = pgTable('lesson_plan_item_grade_history', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  lessonPlanItemId: uuid('lesson_plan_item_id')
+    .notNull()
+    .references(() => lessonPlanItems.id),
+  gradingScaleId: uuid('grading_scale_id').references(() => gradingScales.id),
+  gradeValue: varchar('grade_value', { length: 50 }).notNull(),
+  gradedBy: uuid('graded_by').references(() => users.id),
+  sessionId: uuid('session_id').references(() => lessonSessions.id),
+  notes: text('notes'),
+  createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
+})
+
+export const lessonSessionPlanItems = pgTable('lesson_session_plan_item', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  sessionId: uuid('session_id')
+    .notNull()
+    .references(() => lessonSessions.id),
+  lessonPlanItemId: uuid('lesson_plan_item_id')
+    .notNull()
+    .references(() => lessonPlanItems.id),
+  createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
+})
+
 // --- Type exports ---
 
 export type Instructor = typeof instructors.$inferSelect
@@ -244,3 +268,7 @@ export type LessonPlanSection = typeof lessonPlanSections.$inferSelect
 export type LessonPlanSectionInsert = typeof lessonPlanSections.$inferInsert
 export type LessonPlanItem = typeof lessonPlanItems.$inferSelect
 export type LessonPlanItemInsert = typeof lessonPlanItems.$inferInsert
+export type LessonPlanItemGradeHistory = typeof lessonPlanItemGradeHistory.$inferSelect
+export type LessonPlanItemGradeHistoryInsert = typeof lessonPlanItemGradeHistory.$inferInsert
+export type LessonSessionPlanItem = typeof lessonSessionPlanItems.$inferSelect
+export type LessonSessionPlanItemInsert = typeof lessonSessionPlanItems.$inferInsert