Fix null volunteers array in shift instance JSON responses

Initialize slice helpers with make() instead of var declaration so
empty results serialize as [] instead of null in JSON, fixing the
frontend TypeError on the schedules page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/schedule/schedule.go

@@ -590,7 +590,7 @@ func (s *Store) templateRoles(ctx context.Context, templateID int64) ([]Template
 		return nil, fmt.Errorf("get template roles: %w", err)
 	}
 	defer rows.Close()
-	var roles []TemplateRole
+	roles := make([]TemplateRole, 0)
 	for rows.Next() {
 		var r TemplateRole
 		if err := rows.Scan(&r.ID, &r.TemplateID, &r.RoleName, &r.Count); err != nil {
@@ -608,7 +608,7 @@ func (s *Store) templateVolunteerIDs(ctx context.Context, templateID int64) ([]i
 		return nil, fmt.Errorf("get template volunteers: %w", err)
 	}
 	defer rows.Close()
-	var ids []int64
+	ids := make([]int64, 0)
 	for rows.Next() {
 		var id int64
 		if err := rows.Scan(&id); err != nil {
@@ -631,7 +631,7 @@ func (s *Store) instanceVolunteers(ctx context.Context, instanceID int64) ([]Ins
 		return nil, fmt.Errorf("get instance volunteers: %w", err)
 	}
 	defer rows.Close()
-	var vols []InstanceVolunteer
+	vols := make([]InstanceVolunteer, 0)
 	for rows.Next() {
 		var iv InstanceVolunteer
 		var confirmedAt sql.NullString