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.

lang/en/lang.yaml

@@ -23,6 +23,8 @@ common:
     sort_order: Sort order
     created_at: Created
     none: — none —
+    meeting: For meeting
+    meeting_comment: The dashboard suggests this item before that meeting.
 forms:
     wyklad: Lecture
     cwiczenia: Tutorial

lang/pl/lang.yaml

@@ -23,6 +23,8 @@ common:
     sort_order: Kolejność
     created_at: Utworzono
     none: — brak —
+    meeting: Na zjazd
+    meeting_comment: Pulpit poleca tę pozycję przed tym zjazdem.
 forms:
     wyklad: Wykład
     cwiczenia: Ćwiczenia