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/api/json.go

@@ -204,6 +204,7 @@ type materialListJSON struct {
 	ID          uint       `json:"id"`
 	CourseID    *uint      `json:"course_id"`
 	Course      *courseRef `json:"course"`
+	MeetingID   *uint      `json:"meeting_id"`
 	Title       string     `json:"title"`
 	Type        string     `json:"type"`
 	Source      *string    `json:"source"`
@@ -218,6 +219,7 @@ func newMaterialListJSON(m models.Material, files int64) materialListJSON {
 		ID:         m.ID,
 		CourseID:   m.CourseID,
 		Course:     newCourseRef(m.Course),
+		MeetingID:  m.MeetingID,
 		Title:      m.Title,
 		Type:       m.Type,
 		Source:     m.Source,