downstream: ensure numerics always carry a source prefix

From [1]:

> Distinct from a normal message, a numeric reply MUST contain a
> <source> and use a three-digit numeric as the command.

[1]: https://modern.ircdocs.horse/#numeric-replies

downstream.go

@@ -530,6 +530,12 @@ func (dc *downstreamConn) SendMessage(ctx context.Context, msg *irc.Message) {
 		msg = &msgCopy
 		msg.Prefix = nil
 	}
+	if msg.Prefix == nil && isNumeric(msg.Command) {
+		// Numerics must always carry a source
+		msgCopy := *msg
+		msg = &msgCopy
+		msg.Prefix = dc.srv.prefix()
+	}
 
 	dc.srv.metrics.downstreamOutMessagesTotal.Inc()
 	dc.conn.SendMessage(ctx, msg)