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

@@ -4,12 +4,15 @@ import {
   varchar,
   text,
   time,
+  date,
+  numeric,
   timestamp,
   boolean,
   integer,
   pgEnum,
 } from 'drizzle-orm/pg-core'
 import { users } from './users.js'
+import { accounts, members } from './accounts.js'
 
 // --- Enums ---
 
@@ -57,6 +60,37 @@ export const scheduleSlots = pgTable('schedule_slot', {
   updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
 })
 
+export const enrollmentStatusEnum = pgEnum('enrollment_status', [
+  'active',
+  'paused',
+  'cancelled',
+  'completed',
+])
+
+export const enrollments = pgTable('enrollment', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  memberId: uuid('member_id')
+    .notNull()
+    .references(() => members.id),
+  accountId: uuid('account_id')
+    .notNull()
+    .references(() => accounts.id),
+  scheduleSlotId: uuid('schedule_slot_id')
+    .notNull()
+    .references(() => scheduleSlots.id),
+  instructorId: uuid('instructor_id')
+    .notNull()
+    .references(() => instructors.id),
+  status: enrollmentStatusEnum('status').notNull().default('active'),
+  startDate: date('start_date').notNull(),
+  endDate: date('end_date'),
+  monthlyRate: numeric('monthly_rate', { precision: 10, scale: 2 }),
+  makeupCredits: integer('makeup_credits').notNull().default(0),
+  notes: text('notes'),
+  createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
+  updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
+})
+
 // --- Type exports ---
 
 export type Instructor = typeof instructors.$inferSelect
@@ -65,3 +99,5 @@ export type LessonType = typeof lessonTypes.$inferSelect
 export type LessonTypeInsert = typeof lessonTypes.$inferInsert
 export type ScheduleSlot = typeof scheduleSlots.$inferSelect
 export type ScheduleSlotInsert = typeof scheduleSlots.$inferInsert
+export type Enrollment = typeof enrollments.$inferSelect
+export type EnrollmentInsert = typeof enrollments.$inferInsert