feat: keep the materials index in sync and add bm:materials-reindex

- the materials admin syncs after create and update and removes after
  delete, once the save transaction commits (UseMaterialIndex, set at Boot)
- POST and PUT /api/v1/bm/materials sync after the transaction, DELETE
  removes the document (its statement has no primary key, so no callback)
- bm:materials-reindex [--fresh] [--if-configured] ensures the index and
  syncs every material; it refuses without an engine and runs nightly
  at 04:15 with --if-configured
- TestMaterialSearchAgainstDatabase (Postgres) is written for the final
  verification phase

controllers/api/material_access.go

@@ -322,7 +322,8 @@ func (h *Handlers) UpdateMaterialAccess(w http.ResponseWriter, r *http.Request)
 // applyMaterialAccess writes a visibility and share change onto m inside
 // tx. A material with no author gets userID as author (the claim rule).
 // Only is_private and author_user_id are updated; the shares are replaced
-// when setShares.
+// when setShares. The search index holds none of these, so no caller syncs
+// after it.
 func applyMaterialAccess(tx *gorm.DB, m *models.Material, userID uint, isPrivate *bool, setShares bool, shares []uint) error {
 	if m.AuthorUserID == nil {
 		id := userID