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.