Adjust Server::dynamicAddMedia() and related parts a bit

src/server.h

@@ -94,6 +94,8 @@ struct MediaInfo
 	std::string sha1_digest;
 	// true = not announced in TOCLIENT_ANNOUNCE_MEDIA (at player join)
 	bool no_announce;
+	// if true, this is an ephemeral entry. used by dynamic media.
+	bool ephemeral;
 	// does what it says. used by some cases of dynamic media.
 	bool delete_at_shutdown;
 
@@ -102,6 +104,7 @@ struct MediaInfo
 		path(path_),
 		sha1_digest(sha1_digest_),
 		no_announce(false),
+		ephemeral(false),
 		delete_at_shutdown(false)
 	{
 	}