[schedule] Minor changes

Two minor changes to the conference schedule:
- [ ] On the `event` level add an additional key `url` that can link to an abstract.
- [ ] On the `event` level also support the key `speakers` instead of `authors`.

No file or test is named in the issue. Locate the schedule event handling entry point and inspect how authors are currently read or rendered; then verify both the new url field and speakers alias with the project's existing checks or schedule examples.

Written by the indexing model from the issue text.