A material may be private: only its author and the users it is shared
with see it. Every student read (list, detail, course detail and
counters, the checklist linked material, the token twins) goes through
one visibility fragment. Existing materials stay public with no author.
GET/PUT /_bm/api/v1/materials/{id}/access let the author, a material
manager, toggle the flag and pick the users; the first change of an
author-less material makes the caller its author.
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.
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.
