Avoid an expensive filter on each request

[chintal]

https://github.com/tendril-framework/tendril-filestore/blob/main/src/tendril/filestore/actual.py#L155

Using the list() endpoint for getting filenames and filtering on it produces nothing of value beyond making sure the files exist. However, this filter is going to grow rapidly and making this check on such a generalized query is a terrible idea.

Remove the list() usage here. Even if session management and exception handling aren't robust enough to prevent a desync, have a separate maintenance process which verifies integrity in the background.

Before actually making the change, make sure the 'page' argument isn't actually doing something useful here.