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.
Datepickers on native time.Time / lagoon.Date fields, file uploads on
materials, readings and class sessions managed from the course form
(course_id made nullable).
Courses, meetings, class sessions, deadlines, readings with per-user
progress, materials with tags and file attachments. Timestamps are
stored in UTC and held as Europe/Warsaw text so the admin text inputs
can write them.
