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.

models/materials/fields.yaml

@@ -36,6 +36,13 @@ fields:
         label: jz.bm::lang.common.url
         type: text
         span: right
+    meeting:
+        label: jz.bm::lang.common.meeting
+        type: relation
+        nameFrom: number
+        span: left
+        emptyOption: jz.bm::lang.common.none
+        comment: jz.bm::lang.common.meeting_comment
     tags:
         label: jz.bm::lang.materials.tags
         type: relation