Support featured event images and improve algorithm

Today we noticed that the featured image for upcoming events wasn't showing in the newsletter. [This code](https://github.com/indieweb/this-week/blob/main/generate-events-summary.php#L86) only looks for the `photo` property, which is preferable for **past** events, so we can include a group photo in the newsletter.

For **future** events, though, the newsletter should use the `featured` property instead.

This came up with today's newsletter drafts with some sub-events
* [Beer Garden](https://events.indieweb.org/2025/12/beer-garden-san-diego-ZNZ0Bj32VgNn) which is a sub-event of [IWC SD](https://events.indieweb.org/2025/12/indiewebcamp-san-diego-2025-jhCO6w8CyVHg)
* The parent event was sent as a webmention, so the featured image for IWC SD appears in the responses section with `u-photo` on the sub-event. That's what the newsletter used for those sub-events.
* Newsletter draft screenshot: https://chat-uploads.indieweb.org/files/2025/12/meta-20251212-172454-image_from_ios.jpg

Start with generate-events-summary.php around line 86, where the featured image is selected. Compare the image property used for past events with the featured property needed for future events, including the sub-event example in the issue. Done means upcoming events use featured images while past events retain the existing photo behavior.

Written by the indexing model from the issue text.