feat: materials by zjazd in the student API and admin

The material admin form gets the "Na zjazd" picker, every student
material JSON carries meeting_id, and GET /materials accepts meeting=
inside the published-only scope, so the Pulpit hero can list the
zjazd's materials next to its readings.

controllers/materials.go

@@ -29,11 +29,12 @@ func (materialsAdmin) RequiredPermissions() []string {
 	return []string{PermMaterials}
 }
 
-// AdminFieldRelations binds the course picker and the tags. The files field
+// AdminFieldRelations binds the course and meeting pickers and the tags. The files field
 // is bound by models.Material.AttachRelations.
 func (materialsAdmin) AdminFieldRelations() []cabana.FieldRelationContract {
 	return []cabana.FieldRelationContract{
 		courseRelation("course_id"),
+		meetingRelation(),
 		{
 			Field:             "tags",
 			Kind:              "belongsToMany",