feat: announce material revisions on materials:{id}

- 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

controllers/api/api.go

@@ -78,6 +78,9 @@ type Handlers struct {
 	// means a search (q) answers 503 and the syncs after a write are
 	// skipped.
 	MaterialIndex *classes.MaterialIndexer
+	// Revisions publishes the live heads-up after a write that bumped a
+	// material's revision (D-20). Nil means no live event.
+	Revisions *classes.MaterialRevisionNotifier
 }
 
 func (h *Handlers) now() time.Time {