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.go

@@ -25,7 +25,9 @@ import (
 // plain "?" so users don't have to hunt for the exact red question mark.
 var questionReactions = map[string]bool{
 	"❓":  true, // U+2753 red question mark
+	"❓️": true, // U+2753 + VS16 (emoji presentation)
 	"❔":  true, // U+2754 white question mark
+	"❔️": true, // U+2754 + VS16
 	"⁉":  true, // U+2049 exclamation question mark
 	"⁉️": true, // U+2049 + VS16 (emoji presentation)
 	"🤔":  true, // U+1F914 thinking face