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/material_access.go

@@ -170,6 +170,12 @@ func replaceMaterialShares(tx *gorm.DB, materialID uint, userIDs []uint) error {
 
 // managesMaterials tells whether the user holds jz.bm.access_materials.
 func (h *Handlers) managesMaterials(ctx context.Context, db *gorm.DB, userID uint) (bool, error) {
+	return ManagesMaterials(ctx, db, userID)
+}
+
+// ManagesMaterials tells whether the user holds jz.bm.access_materials. The
+// realtime connection-token guard admits such users (D-20).
+func ManagesMaterials(ctx context.Context, db *gorm.DB, userID uint) (bool, error) {
 	principal, err := userclasses.PrincipalByID(ctx, db, userID)
 	if err != nil {
 		return false, err
@@ -177,6 +183,26 @@ func (h *Handlers) managesMaterials(ctx context.Context, db *gorm.DB, userID uin
 	return principal != nil && cabana.Allows(principal, []string{controllers.PermMaterials}), nil
 }
 
+// CanWatchMaterial tells whether the user may follow material id's live
+// revision channel (D-20): the material is visible to them at now and they
+// may manage it (canManageMaterial). It reads current state on every call;
+// a material they cannot see is false without an error.
+func CanWatchMaterial(ctx context.Context, db *gorm.DB, now time.Time, userID, id uint) (bool, error) {
+	var m models.Material
+	res := visibleMaterials(db.WithContext(ctx), now, userID).Where("jz_bm_materials.id = ?", id).Limit(1).Find(&m)
+	if res.Error != nil {
+		return false, res.Error
+	}
+	if res.RowsAffected == 0 {
+		return false, nil
+	}
+	manager, err := ManagesMaterials(ctx, db, userID)
+	if err != nil {
+		return false, err
+	}
+	return canManageMaterial(m, userID, manager), nil
+}
+
 // canManageMaterial tells whether a material manager may change the
 // material's access: it is theirs, or it has no author yet (the first change
 // makes them its author).