Add video picker and Discord video attachments.

The picker accepts one MP4 or WebM, the thread lightbox zooms photos, and Discord gets the video as a file on a follow-up message so photo embeds still render.

internal/events/bus.go

@@ -40,12 +40,14 @@ func newBus(buffer int, start bool) *Bus {
 // Publish enqueues ev. It never blocks the caller; a full buffer is dropped.
 func (b *Bus) Publish(_ context.Context, ev any) {
 	if b == nil {
+		CallRelease(ev)
 		return
 	}
 	select {
 	case b.ch <- ev:
 	default:
 		log.Printf("events: dropped %T", ev)
+		CallRelease(ev)
 	}
 }