Fix !post in round-robin mode, ❓ reaction VS16, image label

- !post falls back to newest unposted story for the channel when the
  in-memory queue is empty (the steady state under round-robin).
- Accept ❓️/❔️ (U+FE0F variation selector) as question reactions —
  the bare codepoints alone missed clients that render the colored emoji.
- Rewrite Guardian i.guim.co.uk thumbnails to width=1200 so we stop
  rejecting real images as "tracking pixels"; relabel the size warning.
- Log decrypt failures and reactions on events not in post_log so future
  silent drops surface instead of vanishing.

internal/storage/queries.go

@@ -247,6 +247,29 @@ func GetNewestPostableStory(source string) (*Story, error) {
 	return &s, nil
 }
 
+// GetNewestPostableStoryByChannel returns the newest classified story routed
+// to the given channel that has not yet been posted. Used by !post to satisfy
+// on-demand requests in round-robin mode (where the in-memory queue is empty
+// between ticks). Returns (nil, nil) when nothing qualifies.
+func GetNewestPostableStoryByChannel(channel 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 channel = ?
+		   AND guid NOT IN (SELECT guid FROM post_log)
+		 ORDER BY seen_at DESC
+		 LIMIT 1`, channel)
+	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) {