Add lessons Phase 6: lesson plans with curriculum tracking

Structured lesson plans with nested sections and items per enrollment.
Deep create in one request, one-active-per-enrollment constraint,
auto-set startedDate/masteredDate on status transitions, progress %
calculation (skipped items excluded). 8 new tests (84 total).

packages/shared/src/schemas/index.ts

@@ -118,6 +118,10 @@ export {
   LessonSessionUpdateSchema,
   GradingScaleCreateSchema,
   GradingScaleUpdateSchema,
+  LessonPlanItemStatus,
+  LessonPlanCreateSchema,
+  LessonPlanUpdateSchema,
+  LessonPlanItemUpdateSchema,
 } from './lessons.schema.js'
 export type {
   InstructorCreateInput,
@@ -134,4 +138,7 @@ export type {
   LessonSessionUpdateInput,
   GradingScaleCreateInput,
   GradingScaleUpdateInput,
+  LessonPlanCreateInput,
+  LessonPlanUpdateInput,
+  LessonPlanItemUpdateInput,
 } from './lessons.schema.js'