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

@@ -3,6 +3,7 @@ import {
   uuid,
   varchar,
   text,
+  time,
   timestamp,
   boolean,
   integer,
@@ -39,9 +40,28 @@ export const lessonTypes = pgTable('lesson_type', {
   updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
 })
 
+export const scheduleSlots = pgTable('schedule_slot', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  instructorId: uuid('instructor_id')
+    .notNull()
+    .references(() => instructors.id),
+  lessonTypeId: uuid('lesson_type_id')
+    .notNull()
+    .references(() => lessonTypes.id),
+  dayOfWeek: integer('day_of_week').notNull(), // 0=Sunday, 6=Saturday
+  startTime: time('start_time').notNull(),
+  room: varchar('room', { length: 100 }),
+  maxStudents: integer('max_students').notNull().default(1),
+  isActive: boolean('is_active').notNull().default(true),
+  createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
+  updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
+})
+
 // --- Type exports ---
 
 export type Instructor = typeof instructors.$inferSelect
 export type InstructorInsert = typeof instructors.$inferInsert
 export type LessonType = typeof lessonTypes.$inferSelect
 export type LessonTypeInsert = typeof lessonTypes.$inferInsert
+export type ScheduleSlot = typeof scheduleSlots.$inferSelect
+export type ScheduleSlotInsert = typeof scheduleSlots.$inferInsert