feat(classes): weekly recurring schedules with lazy session materialization

class_schedules rules generate concrete session rows for a rolling 28-day
horizon — no cron; materialize runs idempotently on dashboard and public
reads via a partial unique index on (schedule_id, starts_at). Rule delete
stops future sessions; existing sessions keep bookings and overrides.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/db/class_schedules.go

@@ -0,0 +1,101 @@
+package db
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+const classScheduleColumns = "id, tenant_id, template_id, location_id, day_of_week, starts_local::text, active, created_at"
+
+func (r *PGRepository) ListClassSchedulesByTenant(ctx context.Context, tenantID string) ([]ClassScheduleRecord, error) {
+	rows, err := r.pool.Query(ctx, `
+		SELECT `+classScheduleColumns+`
+		FROM class_schedules
+		WHERE tenant_id = $1
+		ORDER BY day_of_week ASC, starts_local ASC
+	`, tenantID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var records []ClassScheduleRecord
+	for rows.Next() {
+		var record ClassScheduleRecord
+		if err := rows.Scan(
+			&record.ID,
+			&record.TenantID,
+			&record.TemplateID,
+			&record.LocationID,
+			&record.DayOfWeek,
+			&record.StartsLocal,
+			&record.Active,
+			&record.CreatedAt,
+		); err != nil {
+			return nil, err
+		}
+		records = append(records, record)
+	}
+	return records, rows.Err()
+}
+
+func (r *PGRepository) CreateClassSchedule(ctx context.Context, params CreateClassScheduleParams) (ClassScheduleRecord, error) {
+	var record ClassScheduleRecord
+	err := r.pool.QueryRow(ctx, `
+		INSERT INTO class_schedules (tenant_id, template_id, location_id, day_of_week, starts_local)
+		VALUES ($1, $2, $3, $4, $5::time)
+		RETURNING `+classScheduleColumns+`
+	`, params.TenantID, params.TemplateID, params.LocationID, params.DayOfWeek, params.StartsLocal).Scan(
+		&record.ID,
+		&record.TenantID,
+		&record.TemplateID,
+		&record.LocationID,
+		&record.DayOfWeek,
+		&record.StartsLocal,
+		&record.Active,
+		&record.CreatedAt,
+	)
+	return record, err
+}
+
+func (r *PGRepository) DeleteClassSchedule(ctx context.Context, tenantID string, scheduleID string) error {
+	tag, err := r.pool.Exec(ctx, `DELETE FROM class_schedules WHERE id = $1 AND tenant_id = $2`, scheduleID, tenantID)
+	if err != nil {
+		return err
+	}
+	if tag.RowsAffected() == 0 {
+		return fmt.Errorf("class schedule not found")
+	}
+	return nil
+}
+
+// MaterializeScheduleSessions writes concrete session rows for each active
+// rule over the next `days` days in the tenant's timezone. The partial unique
+// index on (schedule_id, starts_at) makes this idempotent: sessions already
+// materialized are skipped, and sessions the staff edited or cancelled stay
+// untouched.
+func (r *PGRepository) MaterializeScheduleSessions(ctx context.Context, tenantID string, now time.Time, days int) error {
+	_, err := r.pool.Exec(ctx, `
+		INSERT INTO class_sessions (tenant_id, template_id, location_id, starts_at, ends_at, capacity, schedule_id)
+		SELECT
+			s.tenant_id,
+			s.template_id,
+			s.location_id,
+			(d.day + s.starts_local) AT TIME ZONE t.timezone,
+			(d.day + s.starts_local) AT TIME ZONE t.timezone + make_interval(mins => tpl.duration_minutes),
+			tpl.capacity,
+			s.id
+		FROM class_schedules s
+		INNER JOIN tenants t ON t.id = s.tenant_id
+		INNER JOIN class_templates tpl ON tpl.id = s.template_id
+		CROSS JOIN generate_series(0, $3::int - 1) AS g(i)
+		CROSS JOIN LATERAL (SELECT ($2::timestamptz AT TIME ZONE t.timezone)::date + g.i AS day) d
+		WHERE s.tenant_id = $1
+		  AND s.active
+		  AND EXTRACT(ISODOW FROM d.day)::int = ((s.day_of_week + 6) % 7) + 1
+		  AND (d.day + s.starts_local) AT TIME ZONE t.timezone > $2
+		ON CONFLICT (schedule_id, starts_at) WHERE schedule_id IS NOT NULL DO NOTHING
+	`, tenantID, now, days)
+	return err
+}