Add lessons Phase 5: grading scales with nested levels

Custom grading scales with ordered levels (value, label, numeric score,
color). Supports one-default-per-store constraint, deep create with
nested levels, lookup endpoint for dropdowns, and search/pagination.
12 new tests (76 total lessons tests).

packages/shared/src/schemas/index.ts

@@ -116,6 +116,8 @@ export {
   LessonSessionStatusUpdateSchema,
   LessonSessionNotesSchema,
   LessonSessionUpdateSchema,
+  GradingScaleCreateSchema,
+  GradingScaleUpdateSchema,
 } from './lessons.schema.js'
 export type {
   InstructorCreateInput,
@@ -130,4 +132,6 @@ export type {
   LessonSessionStatusUpdateInput,
   LessonSessionNotesInput,
   LessonSessionUpdateInput,
+  GradingScaleCreateInput,
+  GradingScaleUpdateInput,
 } from './lessons.schema.js'