Add round-robin posting mode

One story per interval_hours (default 4), cycling through enabled sources
in config order. Empty sources are skipped and the pointer advances to
whichever source actually posted. State persists across restarts.

Duplicate-flagged stories now get a _duplicate sentinel channel so they
stay out of the rotation pool alongside _discarded.

main.go

@@ -15,6 +15,7 @@ import (
 	"pete/internal/ingestion"
 	"pete/internal/matrix"
 	"pete/internal/poster"
+	"pete/internal/scheduler"
 	"pete/internal/storage"
 )
 
@@ -94,7 +95,10 @@ func main() {
 	go queue.Start(ctx)
 	slog.Info("post queue started")
 
-	// Build the pipeline callback: classify → enqueue
+	roundRobinMode := cfg.Posting.RoundRobin.Enabled
+
+	// Build the pipeline callback: classify → enqueue (or just classify+store
+	// when round-robin mode is enabled; the scheduler does the enqueueing).
 	processItem := func(ctx context.Context, item *ingestion.FeedItem) {
 		result, err := cls.Classify(ctx, item)
 		if err != nil {
@@ -123,6 +127,9 @@ func main() {
 		storage.MarkClassified(item.GUID, result.Channel, platforms)
 
 		if result.DuplicateOf != nil {
+			// Overwrite with sentinel so the round-robin scheduler doesn't pick
+			// this up later. Immediate-mode posting already returned above.
+			storage.MarkClassified(item.GUID, "_duplicate", "[]")
 			slog.Info("story deduplicated",
 				"guid", item.GUID,
 				"duplicate_of", *result.DuplicateOf,
@@ -130,6 +137,14 @@ func main() {
 			return
 		}
 
+		if roundRobinMode {
+			// Story stays in DB classified+postable; the scheduler will pick
+			// it up on the next rotation tick.
+			slog.Info("story classified, awaiting round-robin tick",
+				"guid", item.GUID, "source", item.Source, "channel", result.Channel)
+			return
+		}
+
 		// Validate image
 		imageURL := ""
 		if item.ImageURL != "" && ingestion.ValidateImageURL(item.ImageURL) {
@@ -154,6 +169,15 @@ func main() {
 	poller.Start(ctx)
 	slog.Info("pollers started")
 
+	if roundRobinMode {
+		rr := scheduler.New(cfg.Sources, cfg.Posting.RoundRobin.IntervalHours, queue)
+		if rr == nil {
+			slog.Warn("round-robin enabled but no sources are enabled; nothing will post")
+		} else {
+			go rr.Start(ctx)
+		}
+	}
+
 	// Run maintenance on startup
 	storage.RunMaintenance(cfg.Storage.RecentWindowHours, cfg.Storage.ClassificationLogDays)