Show one-source events' pictures and play their clips in the News panel

A feed card shows its event's picture whatever the number of sources, so a
lone Telegram post opening with a video gets its preview; the globe's
picture cards still need two. An event picture may carry its post's clip (an
HTTPS MP4 or WebM file): the card marks it with a play sign, and opened, it
plays with controls and sound, loading nothing before. An expired link drops
the clip.

changelog.d/1-news-layer.md

@@ -3,7 +3,9 @@
   in the panel to the last 12, 6, 3 or 1 hours with the hotspot counts read for
   that window, coloured by severity only, with hotspot
   beacons, outlines with disputed areas, picture cards for major events and a
-  continuous feed whose cards fly the globe to the event. The panel's Local
+  continuous feed whose cards fly the globe to the event. A feed card shows its
+  event's picture even when one source reports it, and an open card plays the
+  post's clip on request. The panel's Local
   section lists every report within 500 km of the view below 3,000 km, and the
   map shows those instead of the hotspots' news while it is chosen. Core serves no news itself: set `GEV_NEWS_ORIGIN` to a news
   server and the local server forwards the layer's three read routes there.