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_test.go

@@ -86,7 +86,7 @@ func TestDeleteKeysThumbPrefixIsIDDelimited(t *testing.T) {
 			t.Fatal(err)
 		}
 	}
-	if err := DeleteKeys(ctx, bucket, blobKeysFor(File{ID: 4, DiskName: disk})); err != nil {
+	if err := DeleteKeys(ctx, bucket, BlobKeys(File{ID: 4, DiskName: disk})); err != nil {
 		t.Fatal(err)
 	}
 	for _, key := range doomed {