feat(noitu): add /noitupvp group play

/noitupvp posts the noitu game card in a group and records it as a PvP
card. Everyone who presses Play on that card joins one room: the first
member hosts and starts, 2 to 8 players take 30-second turns, a player
whose turn runs out or who gives up is eliminated, and the last one
standing wins. Each player's points, plus a winner bonus, go to the
card's high-score table, and the bot replies to the card with the result.

The engine now shares one board between the solo game and the new
multiplayer match, so both use the same word rules and scoring.

cmd/server/command_menu_test.go

@@ -74,6 +74,7 @@ func TestCommandDiscovery_AllPublicCommandsHaveSafeMetadata(t *testing.T) {
 		"monkeyd_crawl":        "<url> [font_size]",
 		"monkeyd_tags":         "<url>",
 		"noitu":                "",
+		"noitupvp":             "",
 		"random":               "<option,...>",
 		"addsticker":           "[emoji...]",
 		"alias":                "<name>",