BUG: flatten cache entry in FilestoreBackend.save_entry

.save_entry didn't flatten the new entry when inserting it into the
backend cache, and the cache is reconstructed from _root in every
_load_or_initialize call.  This means once the filestore_backend
fixture is set up, the final entry's children aren't accessible from
the cache until _load_or_initialize is called.

Flattening new entries in .save_entry fixes this issue.

superscore/backends/filestore.py

@@ -264,7 +264,7 @@ def save_entry(self, entry: Entry) -> None:
             if db.get(entry.uuid):
                 raise BackendError("Entry already exists, try updating the entry "
                                    "instead of saving it")
-            db[entry.uuid] = entry
+            self.flatten_and_cache(entry)
             self._root.entries.append(entry)
 
     def update_entry(self, entry: Entry) -> None: