feat: scheduled report digests + per-session capacity (M33)

POST /v1/internal/jobs/reports/dispatch emails each tenant owner a
trailing-window digest (bookings, revenue, per-service and per-staff
breakdown) localized to the tenant locale; zero-activity tenants are
skipped. Metrics cores extracted to tenant-scoped variants so the job
needs no principal.

PATCH /v1/catalog/class-sessions/:id sets a session's seat count with
a confirmed-bookings floor — shrinking below it needs allowOverCapacity,
matching the waitlist-promotion convention. Session rows gain an inline
capacity editor with an overbook confirm state.

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

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

apps/backend/internal/catalog/service.go

@@ -1595,6 +1595,76 @@ func (s *Service) DeleteClassSession(ctx context.Context, principal domain.Princ
 	return s.repo.DeleteClassSession(ctx, sessionID)
 }
 
+// ErrCapacityBelowBooked rejects shrinking a session's capacity under the
+// number of confirmed seats unless the caller passes allowOverCapacity.
+var ErrCapacityBelowBooked = errors.New("capacity below confirmed bookings")
+
+// UpdateClassSession patches a single session — currently only the seat
+// count. Shrinking below confirmed bookings needs an explicit override so a
+// typo cannot silently overbook a session.
+func (s *Service) UpdateClassSession(ctx context.Context, principal domain.Principal, sessionID string, req domain.UpdateClassSessionRequest) (domain.ClassSession, error) {
+	membership, err := s.repo.GetTenantMembershipBySubject(ctx, principal.Subject)
+	if err != nil {
+		return domain.ClassSession{}, ErrTenantMembership
+	}
+	session, err := s.repo.GetClassSessionByID(ctx, sessionID)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return domain.ClassSession{}, ErrClassSessionNotFound
+		}
+		return domain.ClassSession{}, err
+	}
+	if session.TenantID != membership.Tenant.ID {
+		return domain.ClassSession{}, ErrClassSessionNotFound
+	}
+	if req.Capacity == nil {
+		return domain.ClassSession{}, fmt.Errorf("%w: capacity is required", ErrInvalidInput)
+	}
+	if *req.Capacity < 1 || *req.Capacity > 500 {
+		return domain.ClassSession{}, fmt.Errorf("%w: capacity must be 1-500", ErrInvalidInput)
+	}
+
+	sessionBookings, err := s.repo.ListBookingsBySession(ctx, sessionID)
+	if err != nil {
+		return domain.ClassSession{}, err
+	}
+	confirmed := 0
+	for _, booking := range sessionBookings {
+		if booking.Status == "confirmed" {
+			confirmed++
+		}
+	}
+	if *req.Capacity < confirmed && !req.AllowOverCapacity {
+		return domain.ClassSession{}, fmt.Errorf("%w: %d confirmed seats", ErrCapacityBelowBooked, confirmed)
+	}
+
+	rec, err := s.repo.UpdateClassSessionCapacity(ctx, sessionID, *req.Capacity)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return domain.ClassSession{}, ErrClassSessionNotFound
+		}
+		return domain.ClassSession{}, err
+	}
+
+	out := domain.ClassSession{
+		ID:            rec.ID,
+		TenantID:      rec.TenantID,
+		TemplateID:    rec.TemplateID,
+		TemplateTitle: rec.Title,
+		LocationID:    rec.LocationID,
+		StartsAt:      rec.StartsAt,
+		EndsAt:        rec.EndsAt,
+		Capacity:      int(rec.Capacity),
+		BookedCount:   confirmed,
+	}
+	if rec.LocationID != nil {
+		if location, err := s.repo.GetLocationByID(ctx, *rec.LocationID); err == nil {
+			out.LocationName = location.Name
+		}
+	}
+	return out, nil
+}
+
 func (s *Service) requireTenantClassTemplate(ctx context.Context, tenantID, templateID string) error {
 	template, err := s.repo.GetClassTemplateByID(ctx, templateID)
 	if err != nil {