Delete expired Solid Notifications channels every day

At the moment, expired notifications channels (with a `endAt` date in the past) are closed only on server start, on the `loadChannelsFromDb` method.

This may leave hundreds of expired channels in the triple store, creating confusion.

We should add a cron to delete those expired channels every day.

Start at the loadChannelsFromDb method, which currently closes channels only during server startup, and trace how expired channels are represented in the triple store. Identify the project's existing scheduling entry point, then verify that a daily run removes channels whose endAt date is in the past without affecting active channels.

Written by the indexing model from the issue text.