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.

internal/storage/queries.go

@@ -1,7 +1,9 @@
 package storage
 
 import (
+	"database/sql"
 	"encoding/json"
+	"errors"
 	"log/slog"
 	"time"
 )
@@ -221,6 +223,55 @@ func InsertReaction(postGUID, channel, eventID, emoji, userID string, reactedAt
 		postGUID, channel, eventID, emoji, userID, reactedAt)
 }
 
+// GetNewestPostableStory returns the newest classified story from a source
+// that has a real (non-sentinel) channel and has not yet been posted (no
+// row in post_log). Returns (nil, nil) when nothing qualifies.
+func GetNewestPostableStory(source string) (*Story, error) {
+	row := Get().QueryRow(
+		`SELECT guid, headline, lede, image_url, article_url, source, feed_hint, platforms, channel, seen_at
+		 FROM stories
+		 WHERE classified = 1
+		   AND source = ?
+		   AND channel IS NOT NULL
+		   AND channel NOT IN ('_discarded', '_duplicate')
+		   AND guid NOT IN (SELECT guid FROM post_log)
+		 ORDER BY seen_at DESC
+		 LIMIT 1`, source)
+	var s Story
+	if err := row.Scan(&s.GUID, &s.Headline, &s.Lede, &s.ImageURL, &s.ArticleURL, &s.Source, &s.FeedHint, &s.Platforms, &s.Channel, &s.SeenAt); err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	return &s, nil
+}
+
+// GetRoundRobinState returns the last source posted by the round-robin
+// scheduler and the timestamp of that tick. Empty string + 0 if no state yet.
+func GetRoundRobinState() (lastSource string, lastTickAt int64, err error) {
+	row := Get().QueryRow(`SELECT last_source, last_tick_at FROM round_robin_state WHERE id = 1`)
+	var ls *string
+	if scanErr := row.Scan(&ls, &lastTickAt); scanErr != nil {
+		if errors.Is(scanErr, sql.ErrNoRows) {
+			return "", 0, nil
+		}
+		return "", 0, scanErr
+	}
+	if ls != nil {
+		lastSource = *ls
+	}
+	return lastSource, lastTickAt, nil
+}
+
+// SetRoundRobinState persists the last-posted source and tick timestamp.
+func SetRoundRobinState(lastSource string, tickAt int64) {
+	exec("set round_robin_state",
+		`INSERT INTO round_robin_state (id, last_source, last_tick_at) VALUES (1, ?, ?)
+		 ON CONFLICT(id) DO UPDATE SET last_source = excluded.last_source, last_tick_at = excluded.last_tick_at`,
+		lastSource, tickAt)
+}
+
 // MarshalPlatforms converts a string slice to a JSON array string for storage.
 func MarshalPlatforms(platforms []string) string {
 	if len(platforms) == 0 {

internal/storage/schema.go

@@ -40,6 +40,12 @@ CREATE TABLE IF NOT EXISTS post_log (
     posted_at     INTEGER NOT NULL
 );
 
+CREATE TABLE IF NOT EXISTS round_robin_state (
+    id           INTEGER PRIMARY KEY CHECK (id = 1),
+    last_source  TEXT,
+    last_tick_at INTEGER NOT NULL DEFAULT 0
+);
+
 CREATE TABLE IF NOT EXISTS reactions (
     id         INTEGER PRIMARY KEY AUTOINCREMENT,
     post_guid  TEXT NOT NULL,