feat: zjazd (meeting) on readings and materials

The Pulpit hero lists what to read for the current or next zjazd, so
readings and materials get a nullable meeting_id (FK to jz_bm_meetings,
ON DELETE SET NULL, indexed). The reading admin form gets a "Na zjazd"
picker, the student reading JSON carries meeting_id, and the management
/readings records filter and validate it.

controllers/api/json.go

@@ -173,6 +173,7 @@ type readingJSON struct {
 	ID         uint       `json:"id"`
 	CourseID   uint       `json:"course_id"`
 	Course     *courseRef `json:"course"`
+	MeetingID  *uint      `json:"meeting_id"`
 	Title      string     `json:"title"`
 	Author     *string    `json:"author"`
 	Source     *string    `json:"source"`