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>

Makefile

@@ -7,9 +7,11 @@ TEX_FILE_BACK := brevetkarte-rueckseite.tex
 PDF_FILE_PERSONALIZED := brevetkarte-personalized.pdf
 PDF_FILE_BLANKO := brevetkarte-blanko.pdf
 PDF_FILE_BACK := brevetkarte-rueckseite.pdf
+PDF_FILE_DUPLEX := brevetkarte-duplex.pdf
+PDF_FILE_BLANKO_DUPLEX := brevetkarte-blanko-duplex.pdf
 CSV_FILE := export_brevetcard.csv
 
-.PHONY: all build clean build-image build-back generate build-personalized build-blanko run shell help
+.PHONY: all build clean build-image build-back generate build-personalized build-blanko build-duplex build-blanko-duplex run shell help
 
 # Default target
 all: build-personalized build-blanko
@@ -75,6 +77,24 @@ build-blanko: build-image
 		pdflatex -interaction=nonstopmode $(TEX_FILE_BACK)
 	@echo "PDF generated: $(PDF_FILE_BACK)"
 
+# Build duplex PDF (personalized front + back interleaved for duplex printing)
+build-duplex: build-personalized
+	@echo "Combining front and back for duplex printing..."
+	docker run --rm \
+		-v $(PWD):/workspace \
+		$(IMAGE_NAME) \
+		pdftk A=$(PDF_FILE_PERSONALIZED) B=$(PDF_FILE_BACK) shuffle A B output $(PDF_FILE_DUPLEX)
+	@echo "PDF generated: $(PDF_FILE_DUPLEX)"
+
+# Build duplex PDF (blank front + back interleaved for duplex printing)
+build-blanko-duplex: build-blanko
+	@echo "Combining blank front and back for duplex printing..."
+	docker run --rm \
+		-v $(PWD):/workspace \
+		$(IMAGE_NAME) \
+		pdftk A=$(PDF_FILE_BLANKO) B=$(PDF_FILE_BACK) shuffle A B output $(PDF_FILE_BLANKO_DUPLEX)
+	@echo "PDF generated: $(PDF_FILE_BLANKO_DUPLEX)"
+
 # Run container interactively
 run:
 	docker run --rm -it \
@@ -92,7 +112,7 @@ shell:
 # Clean generated files
 clean:
 	@echo "Cleaning generated files..."
-	rm -f *.aux *.log *.out *.toc *.pdf brevetkarte-personalized.tex brevetkarte-blanko.tex brevetkarte-rueckseite.tex
+	rm -f *.aux *.log *.out *.toc *.pdf brevetkarte-personalized.tex brevetkarte-blanko.tex brevetkarte-rueckseite.tex $(PDF_FILE_DUPLEX) $(PDF_FILE_BLANKO_DUPLEX)
 
 # Clean everything including Docker image
 clean-all: clean
@@ -112,6 +132,8 @@ help:
 	@echo "  make generate           - Generate tex files from CSV + event.yml"
 	@echo "  make build-personalized - Generate and compile front + back side PDFs"
 	@echo "  make build-blanko       - Generate and compile blank card (no CSV needed)"
+	@echo "  make build-duplex       - Build duplex PDF (front+back interleaved)"
+	@echo "  make build-blanko-duplex - Build blank duplex PDF"
 	@echo "  make build-back         - Compile back side PDF only (after generate)"
 	@echo "  make shell              - Open interactive shell in container"
 	@echo "  make clean              - Remove generated files (aux, log, pdf)"