Move expiry logic to event model

Ensures that a meetup that has just started is displayed for two hours even if we clear the cache with a deployment before the two hours have passed; also cleans the code

site/controllers/meet.php

@@ -8,7 +8,7 @@
 
 	// expire the cache 2h after the next upcoming event starts
 	if ($next = $upcoming->sortBy('date', 'asc')->first()) {
-		$time = $next->datetime()->getTimestamp() + (60 * 60 * 2);
+		$time = $next->expiryTime();
 		$kirby->response()->expires($time);
 		$kirby->response()->header('Expires', gmdate('D, d M Y H:i:s T', $time));
 	}

site/models/event.php

@@ -20,6 +20,12 @@ public function datetime(): DateTime
 		);
 	}
 
+	public function expiryTime(): int
+	{
+		// expire each event two hours after it started
+		return $this->datetime()->getTimestamp() + (60 * 60 * 2);
+	}
+
 	public function icon(): string
 	{
 		return parent::icon()->or(match (true) {
@@ -35,7 +41,7 @@ public function isMeetup(): bool
 
 	public function isUpcoming(): bool
 	{
-		return $this->datetime()->getTimestamp() >= time();
+		return $this->expiryTime() >= time();
 	}
 
 	public function shortTitle(): Field