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>

generate_cards.py

@@ -59,7 +59,7 @@ def apply_event_placeholders(text, config):
     return text
 
 
-def generate_backside(template, config):
+def generate_backside(template, config, num_pages=1):
     """Fill back side template with cell content from event config."""
     cells = config.get('backside', {})
     result = template
@@ -71,7 +71,18 @@ def generate_backside(template, config):
             if content is None:
                 content = ""
             result = result.replace(placeholder, content.strip())
-    return result
+
+    if num_pages <= 1:
+        return result
+
+    # Repeat body between \begin{document} and \end{document} num_pages times
+    begin_marker = '\\begin{document}\n'
+    end_marker = '\\end{document}'
+    begin_idx = result.find(begin_marker) + len(begin_marker)
+    end_idx = result.rfind(end_marker)
+    preamble = result[:begin_idx]
+    body = result[begin_idx:end_idx].rstrip()
+    return preamble + ('\n\n\\newpage\n\n').join([body] * num_pages) + '\n' + result[end_idx:]
 
 
 def generate_card_from_template(template, data):
@@ -142,7 +153,7 @@ def main():
         blanko_output_file.write_text(blanko_output, encoding='utf-8')
         print(f"Generated {blanko_output_file}")
         backside_template = backside_template_file.read_text(encoding='utf-8')
-        backside_output = generate_backside(backside_template, event_config)
+        backside_output = generate_backside(backside_template, event_config, num_pages=1)
         backside_output_file.write_text(backside_output, encoding='utf-8')
         print(f"Generated {backside_output_file}")
         return
@@ -179,10 +190,11 @@ def main():
     output_file.write_text(''.join(document_parts), encoding='utf-8')
     print(f"Generated {output_file}")
 
-    # Generate personalized back side
+    # Generate personalized back side (one page per front page for duplex)
+    num_pages = (len(participants) + 1) // 2
     print(f"Reading back side template from {backside_template_file}...")
     backside_template = backside_template_file.read_text(encoding='utf-8')
-    backside_output = generate_backside(backside_template, event_config)
+    backside_output = generate_backside(backside_template, event_config, num_pages=num_pages)
     backside_output_file.write_text(backside_output, encoding='utf-8')
     print(f"Generated {backside_output_file}")