Add Phase 4b: instructor blocked dates, store closures, and substitute instructors

- New tables: instructor_blocked_date, store_closure (migration 0034)
- substitute_instructor_id column added to lesson_session
- Session generation skips blocked instructor dates and store closure periods
- Substitute assignment validates sub is not blocked and has no conflicting slot
- Routes: POST/GET/DELETE /instructors/:id/blocked-dates, POST/GET/DELETE /store-closures
- 15 new integration tests covering blocked dates, store closures, and sub validation

packages/shared/src/schemas/index.ts

@@ -122,6 +122,8 @@ export {
   LessonPlanCreateSchema,
   LessonPlanUpdateSchema,
   LessonPlanItemUpdateSchema,
+  InstructorBlockedDateCreateSchema,
+  StoreClosureCreateSchema,
 } from './lessons.schema.js'
 export type {
   InstructorCreateInput,
@@ -141,4 +143,6 @@ export type {
   LessonPlanCreateInput,
   LessonPlanUpdateInput,
   LessonPlanItemUpdateInput,
+  InstructorBlockedDateCreateInput,
+  StoreClosureCreateInput,
 } from './lessons.schema.js'