Extract and insert 97 images from reference PDF into songs

Extract images from the CL6 PDF using pdfimages, map them to songs
via page-to-CL number matching, and insert \songimage commands.
Add insert-images.py script for repeatable extraction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

songs/es-liegt-etwas-auf-den-strassen.tex

@@ -30,4 +30,6 @@ die Drossel weiß meiner Sehnsucht süßesten Reim, \\
 und alle Straßen im Lande sagen: „Kehr heim!“
 \end{verse}
 
+\songimage{images/img-053.png}
+
 \end{song}