- every revision bump (API writes and /bmadmin saves) enqueues
updated.jz.bm.material with {material_id, revision} in a savepoint
- the materials namespace authorizer allows a subscribe only to a
visible material the user may manage
- api.ManagesMaterials and api.CanWatchMaterial
- add jz_bm_materials.revision (default 1), read-only for GORM
- bump it only on content changes (models.ReviseMaterial)
- PUT /materials/{id} locks the row and answers 409 to a stale revision
- management answers carry revision and updated_at, the student detail
carries revision