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/bookings/service.go

@@ -160,6 +160,11 @@ func (s *Service) Availability(ctx context.Context, tenantSlug string, filter Av
 		}, nil
 	}
 
+	// Recurring schedules write their concrete sessions here so the public
+	// booking page sees them without a background job. Idempotent.
+	if err := s.repo.MaterializeScheduleSessions(ctx, tenant.ID, now, 28); err != nil {
+		return domain.PublicAvailability{}, err
+	}
 	classSessions, err := s.repo.ListClassSessionsByTenant(ctx, tenant.ID, windowStart, 8)
 	if err != nil {
 		return domain.PublicAvailability{}, err