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/tracker.go

@@ -32,7 +32,8 @@ func SetReactionCallback(fn ReactionCallback) {
 func HandleReaction(roomID id.RoomID, eventID id.EventID, targetEventID id.EventID, emoji string, userID id.UserID) {
 	guid, channel, found := storage.LookupPostGUID(string(targetEventID))
 	if !found {
-		// Reaction on a message we didn't post — ignore
+		slog.Info("reaction on event not in post_log, ignoring",
+			"target_event", targetEventID, "emoji", emoji, "user", userID)
 		return
 	}