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/json.go

@@ -241,11 +241,13 @@ func newMaterialListJSON(m models.Material, files int64) materialListJSON {
 
 // materialDetailJSON is a published material the caller may see, with its
 // body and files. CanManage is true for the caller who may change its access.
+// Revision is the content revision an edit sends back.
 type materialDetailJSON struct {
 	materialListJSON
 	Body      *string    `json:"body"`
 	Files     []fileJSON `json:"files"`
 	CanManage bool       `json:"can_manage"`
+	Revision  uint       `json:"revision"`
 }
 
 // fileJSON is one attached file.