Round-robin: rotate by channel instead of by source

Source-keyed rotation skewed toward whichever channel had the most
feeds (4 of 7 sources routed to politics, so politics dominated the
rotation). Channel-keyed rotation guarantees variety regardless of
feed counts.

Schema: round_robin_state.last_source -> last_channel, added via
addColumnIfMissing so existing DBs migrate in place.

main.go

@@ -214,9 +214,13 @@ func main() {
 	slog.Info("pollers started")
 
 	if roundRobinMode {
-		rr := scheduler.New(cfg.Sources, cfg.Posting.RoundRobin.IntervalHours, queue)
+		channelNames := make([]string, 0, len(cfg.Matrix.Channels))
+		for name := range cfg.Matrix.Channels {
+			channelNames = append(channelNames, name)
+		}
+		rr := scheduler.New(channelNames, cfg.Posting.RoundRobin.IntervalHours, queue)
 		if rr == nil {
-			slog.Warn("round-robin enabled but no sources are enabled; nothing will post")
+			slog.Warn("round-robin enabled but no channels are configured; nothing will post")
 		} else {
 			go rr.Start(ctx)
 		}