feat: revision-check material writes

- 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

controllers/api/materials.go

@@ -234,6 +234,7 @@ func (h *Handlers) Material(w http.ResponseWriter, r *http.Request) {
 		materialListJSON: newMaterialListJSON(m, int64(len(files))),
 		Body:             m.Body,
 		Files:            make([]fileJSON, 0, len(files)),
+		Revision:         m.Revision,
 	}
 	for i := range files {
 		f := &files[i]