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/explainer/explainer_test.go

@@ -95,7 +95,7 @@ func TestFormatSummary_NoBulletsPassthrough(t *testing.T) {
 }
 
 func TestIsQuestionReaction(t *testing.T) {
-	want := []string{"❓", "❔", "⁉", "⁉️", "🤔", "?", "？"}
+	want := []string{"❓", "❓️", "❔", "❔️", "⁉", "⁉️", "🤔", "?", "？"}
 	for _, k := range want {
 		if !IsQuestionReaction(k) {
 			t.Errorf("expected %q to trigger explain", k)