The management save/update body takes is_private, author_user_id and
share_user_ids (absent keeps each; a new material gets the caller as
author), and the managed material shows them. New routes PATCH
/materials/{id}/visibility, PUT /materials/{id}/shares and GET /users,
with the MCP tools bm_manage_set_material_visibility,
bm_manage_set_material_shares and bm_manage_list_users. The /bmadmin
form gets the private switch, the author picker and shared_with.
Seven admin controllers in a "Studia BM" navigation group, guarded by
jz.bm.access_courses, jz.bm.access_schedule and jz.bm.access_materials,
with Polish and English labels. Date and time fields are checked by the
models before a save and answered as 422 on the field.
