feat: manage material visibility and shares

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.

plugin_test.go

@@ -166,6 +166,9 @@ func TestRoutes(t *testing.T) {
 		"POST /api/v1/bm/materials/{id}/files/inline",
 		"POST /api/v1/bm/materials/{id}/files/upload-url",
 		"DELETE /api/v1/bm/materials/{id}/files/{file}",
+		"PATCH /api/v1/bm/materials/{id}/visibility",
+		"PUT /api/v1/bm/materials/{id}/shares",
+		"GET /api/v1/bm/users",
 		"GET /api/v1/bm/courses",
 		"GET /api/v1/bm/courses/{id}",
 		"POST /api/v1/bm/courses",