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/poster/queue.go

@@ -195,6 +195,13 @@ func (q *Queue) drainChannel(channel string) {
 	q.postItem(item)
 }
 
+// PostNow sends a story immediately, bypassing the in-memory queue and all
+// pacing limits. Last-mile canonical-URL dedup still applies. Used by !post
+// to satisfy on-demand requests with stories pulled directly from storage.
+func (q *Queue) PostNow(item QueueItem) {
+	q.postItem(item)
+}
+
 func (q *Queue) postItem(item QueueItem) {
 	// Last-mile dedup: if this canonical URL was already posted to this channel
 	// within the cooldown window, drop silently. Catches "same article, different