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/matrix/client.go

@@ -133,6 +133,13 @@ func New(cfg config.MatrixConfig) (*Client, error) {
 		return nil, fmt.Errorf("init crypto helper: %w", err)
 	}
 
+	// Surface decrypt failures — the default is a silent no-op, which hides
+	// missing-megolm-session errors (the usual cause of "Pete ignores reactions").
+	ch.DecryptErrorCallback = func(evt *event.Event, err error) {
+		slog.Warn("matrix: failed to decrypt incoming event",
+			"room", evt.RoomID, "event_id", evt.ID, "sender", evt.Sender, "err", err)
+	}
+
 	// LoginAs enables the cryptohelper to re-login if the token expires
 	ch.LoginAs = &mautrix.ReqLogin{
 		Type: mautrix.AuthTypePassword,