make digest sending a hourly cron job

classes/RSSUtils.php

@@ -285,9 +285,6 @@ class RSSUtils {
 			self::housekeeping_user($owner_uid);
 		}
 
-		// Send feed digests by email if needed.
-		Digest::send_headlines_digests();
-
 		return $nf;
 	}
 
@@ -1775,6 +1772,14 @@ class RSSUtils {
 				return 0;
 			}
 		);
+
+		PluginHost::getInstance()->add_scheduled_task('send_headlines_digests', '@hourly',
+			function() {
+				Digest::send_headlines_digests();
+
+				return 0;
+			}
+		);
 	}
 
 	static function housekeeping_common(): void {