feat(12.2-01): add deferred bindings, guarded upload store and purge

- deferred_bindings migration set under summercms.deferred with backend_user_id
- lagoon.DeferredBind/Unbind/Bindings/Forget/Slaves scoped by DeferredKey
- lagoon.PurgeDeferred with SKIP LOCKED batches and after-commit blob deletes
- attach.Store with the ported image guard, extension and MIME limits
- attach.Relation, attach.HasRelations, attach.BlobKeys, File.ThumbKey
- lagoon README and attachments docs

modules/lagoon/attach/file.go

@@ -107,7 +107,10 @@ func init() {
 	Register(&File{})
 }
 
-func blobKeysFor(f File) []string {
+// BlobKeys returns the blob keys of f: the original's partitioned key and
+// the thumb_<id>_ prefix of its thumbnails. DeleteKeys treats the second as
+// a prefix, so passing both removes the original and every thumbnail.
+func BlobKeys(f File) []string {
 	part := PartitionDirectory(f.DiskName)
 	return []string{
 		part + f.DiskName,
@@ -141,7 +144,7 @@ func DeleteForOwner(tx *gorm.DB, owner Owner, ownerID string, afterCommit func(b
 	}
 	var keys []string
 	for _, f := range files {
-		keys = append(keys, blobKeysFor(f)...)
+		keys = append(keys, BlobKeys(f)...)
 	}
 	if err := tx.Where("attachment_type = ? AND attachment_id = ?", morph, ownerID).Delete(&File{}).Error; err != nil {
 		return err