Add lessons Phase 4: lesson sessions with hybrid calendar generation

Individual lesson occurrences generated from schedule slot patterns.
Idempotent session generation with configurable rolling window.
Post-lesson notes workflow with auto-set notesCompletedAt. Status
tracking (scheduled/attended/missed/makeup/cancelled) and date/time
filtering. 13 new tests (64 total lessons tests).

packages/backend/src/db/schema/lessons.ts

@@ -91,6 +91,35 @@ export const enrollments = pgTable('enrollment', {
   updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
 })
 
+export const lessonSessionStatusEnum = pgEnum('lesson_session_status', [
+  'scheduled',
+  'attended',
+  'missed',
+  'makeup',
+  'cancelled',
+])
+
+export const lessonSessions = pgTable('lesson_session', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  enrollmentId: uuid('enrollment_id')
+    .notNull()
+    .references(() => enrollments.id),
+  scheduledDate: date('scheduled_date').notNull(),
+  scheduledTime: time('scheduled_time').notNull(),
+  actualStartTime: time('actual_start_time'),
+  actualEndTime: time('actual_end_time'),
+  status: lessonSessionStatusEnum('status').notNull().default('scheduled'),
+  instructorNotes: text('instructor_notes'),
+  memberNotes: text('member_notes'),
+  homeworkAssigned: text('homework_assigned'),
+  nextLessonGoals: text('next_lesson_goals'),
+  topicsCovered: text('topics_covered').array(),
+  makeupForSessionId: uuid('makeup_for_session_id'),
+  notesCompletedAt: timestamp('notes_completed_at', { withTimezone: true }),
+  createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
+  updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
+})
+
 // --- Type exports ---
 
 export type Instructor = typeof instructors.$inferSelect
@@ -101,3 +130,5 @@ export type ScheduleSlot = typeof scheduleSlots.$inferSelect
 export type ScheduleSlotInsert = typeof scheduleSlots.$inferInsert
 export type Enrollment = typeof enrollments.$inferSelect
 export type EnrollmentInsert = typeof enrollments.$inferInsert
+export type LessonSession = typeof lessonSessions.$inferSelect
+export type LessonSessionInsert = typeof lessonSessions.$inferInsert