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.

internal/storage/db.go

@@ -81,6 +81,7 @@ func runMigrations(d *sql.DB) error {
 	addColumnIfMissing(d, "stories", "url_canonical", "TEXT")
 	addColumnIfMissing(d, "stories", "headline_norm", "TEXT")
 	addColumnIfMissing(d, "post_log", "url_canonical", "TEXT")
+	addColumnIfMissing(d, "round_robin_state", "last_channel", "TEXT")
 
 	// FTS5 virtual tables don't support IF NOT EXISTS reliably.
 	// Check sqlite_master before creating.

internal/storage/queries.go

@@ -270,29 +270,29 @@ func GetNewestPostableStoryByChannel(channel string) (*Story, error) {
 	return &s, nil
 }
 
-// GetRoundRobinState returns the last source posted by the round-robin
+// GetRoundRobinState returns the last channel 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 {
+func GetRoundRobinState() (lastChannel string, lastTickAt int64, err error) {
+	row := Get().QueryRow(`SELECT last_channel, last_tick_at FROM round_robin_state WHERE id = 1`)
+	var lc *string
+	if scanErr := row.Scan(&lc, &lastTickAt); scanErr != nil {
 		if errors.Is(scanErr, sql.ErrNoRows) {
 			return "", 0, nil
 		}
 		return "", 0, scanErr
 	}
-	if ls != nil {
-		lastSource = *ls
+	if lc != nil {
+		lastChannel = *lc
 	}
-	return lastSource, lastTickAt, nil
+	return lastChannel, lastTickAt, nil
 }
 
-// SetRoundRobinState persists the last-posted source and tick timestamp.
-func SetRoundRobinState(lastSource string, tickAt int64) {
+// SetRoundRobinState persists the last-posted channel and tick timestamp.
+func SetRoundRobinState(lastChannel 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)
+		`INSERT INTO round_robin_state (id, last_channel, last_tick_at) VALUES (1, ?, ?)
+		 ON CONFLICT(id) DO UPDATE SET last_channel = excluded.last_channel, last_tick_at = excluded.last_tick_at`,
+		lastChannel, tickAt)
 }
 
 // MarshalPlatforms converts a string slice to a JSON array string for storage.

internal/storage/schema.go

@@ -42,7 +42,7 @@ CREATE TABLE IF NOT EXISTS post_log (
 
 CREATE TABLE IF NOT EXISTS round_robin_state (
     id           INTEGER PRIMARY KEY CHECK (id = 1),
-    last_source  TEXT,
+    last_channel TEXT,
     last_tick_at INTEGER NOT NULL DEFAULT 0
 );