Add lessons Phase 2: schedule slots with conflict detection

Recurring weekly time slots linking instructors to lesson types.
Includes day/time overlap detection, instructor and day-of-week
filtering, and 17 new integration tests (40 total lessons tests).

packages/shared/src/schemas/index.ts

@@ -106,10 +106,14 @@ export {
   InstructorUpdateSchema,
   LessonTypeCreateSchema,
   LessonTypeUpdateSchema,
+  ScheduleSlotCreateSchema,
+  ScheduleSlotUpdateSchema,
 } from './lessons.schema.js'
 export type {
   InstructorCreateInput,
   InstructorUpdateInput,
   LessonTypeCreateInput,
   LessonTypeUpdateInput,
+  ScheduleSlotCreateInput,
+  ScheduleSlotUpdateInput,
 } from './lessons.schema.js'