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.

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

apps/backend/internal/db/services.go

@@ -138,7 +138,7 @@ func (r *PGRepository) ListAvailabilityRulesByTenant(ctx context.Context, tenant
 
 func (r *PGRepository) ListClassSessionsByTenant(ctx context.Context, tenantID string, from time.Time, limit int) ([]ClassSessionRecord, error) {
 	rows, err := r.pool.Query(ctx, `
-		SELECT cs.id, cs.tenant_id, cs.template_id, cs.location_id, ct.title, cs.starts_at, cs.ends_at, cs.capacity
+		SELECT cs.id, cs.tenant_id, cs.template_id, cs.location_id, ct.title, cs.starts_at, cs.ends_at, cs.capacity, cs.schedule_id
 		FROM class_sessions cs
 		INNER JOIN class_templates ct ON ct.id = cs.template_id
 		WHERE cs.tenant_id = $1 AND cs.starts_at >= $2
@@ -162,6 +162,7 @@ func (r *PGRepository) ListClassSessionsByTenant(ctx context.Context, tenantID s
 			&record.StartsAt,
 			&record.EndsAt,
 			&record.Capacity,
+			&record.ScheduleID,
 		); err != nil {
 			return nil, err
 		}