Skip already-processed ZIPs by sampling first JPEG comment

Extracts one JPEG from each ZIP to check for the processing comment
before doing a full extract/resize/rezip cycle. Shared constants moved
to config.sh as single source of truth. Trap and cleanup extended to
cover the sample temp file.

config.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+# Shared configuration for image processing scripts
+
+MAX_PIXELS=8000000
+MAGICK_COMMENT="magick:resize=8mp,quality=85,sampling=4:2:0"