Add duplex PDF build targets with pdftk page interleaving

- Add pdftk-java to Docker image for PDF page manipulation
- Extend generate_backside() to repeat back content num_pages times so
  front and back PDFs have matching page counts for pdftk shuffle
- Add build-duplex and build-blanko-duplex Makefile targets that combine
  front and back PDFs with alternating pages (front1, back1, front2, back2)
- Document duplex targets in README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -7,6 +7,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     make \
     python3 \
     python3-yaml \
+    pdftk-java \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /workspace