Add PDF-based document generation with LibreOffice; fix line numbering and margins

- Add LibreOffice Writer and DejaVu fonts to Dockerfile for PDF generation
- Add PyPDF2 and reportlab to requirements.txt
- Refactor email_sender.py:
  - Enforce 1-inch margins on all sides
  - Isolate line numbering to transcript section only
  - Add generate_pdf_documents() to build:
    - TRANSCRIPT.pdf (cover + transcript)
    - SUMMARY.pdf (cover + summary)
    - COMBINED.pdf (transcript cover + summary + TRANSCRIPT header + transcript)
  - Add page numbers (bottom-right) to all PDFs via reportlab
- Update tasks.py:
  - Use generate_pdf_documents() after creating DOCX files
  - Attach source JSON, MD files, and compiled PDFs in success email
- Add test_docx_generation.py for transcript/summary/combined DOCX testing

requirements.txt

@@ -6,3 +6,5 @@ PyYAML>=6.0
 celery[redis]>=5.3.0
 redis>=5.0.0
 python-docx>=1.1.0
+PyPDF2>=3.0.0
+reportlab>=4.0.0