fix(pdf): replace fpdf2 with weasyprint for proper md→PDF rendering

fpdf2's built-in Helvetica was Latin-1 only and required manual line-by-line
markdown parsing, causing indentation artifacts at speaker changes. weasyprint
renders via HTML+CSS so layout, wrapping, and Unicode all work correctly.

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

requirements.txt

@@ -3,4 +3,4 @@ uvicorn[standard]==0.32.1
 requests==2.32.3
 python-multipart==0.0.12
 markdown==3.7
-fpdf2==2.8.3
+weasyprint>=62.3