Add lessons Phase 3: enrollments with capacity and time conflict checks

Links members to schedule slots via enrollments. Enforces max_students
capacity on slots and prevents members from double-booking the same
day/time. Supports status transitions and filtering. 11 new tests
(51 total lessons tests).

packages/shared/src/schemas/index.ts

@@ -108,6 +108,10 @@ export {
   LessonTypeUpdateSchema,
   ScheduleSlotCreateSchema,
   ScheduleSlotUpdateSchema,
+  EnrollmentStatus,
+  EnrollmentCreateSchema,
+  EnrollmentUpdateSchema,
+  EnrollmentStatusUpdateSchema,
 } from './lessons.schema.js'
 export type {
   InstructorCreateInput,
@@ -116,4 +120,7 @@ export type {
   LessonTypeUpdateInput,
   ScheduleSlotCreateInput,
   ScheduleSlotUpdateInput,
+  EnrollmentCreateInput,
+  EnrollmentUpdateInput,
+  EnrollmentStatusUpdateInput,
 } from './lessons.schema.js'