feat(pdf): add welcome-letter PDF generation and tests

[liocle]

• Add PDF generation dependencies

  • Installed WeasyPrint (and its transitive deps: pydyf, tinycss2, etc.)
  • Updated app/requirements.txt

• Implement welcome-letter generation

  • Created app/pdf/generate_welcome_letter.py
  • Added /members/{member_id}/generate_welcome_letter route in app/api/routes_member.py
  • Generates a PDF file named welcome_letter_<reference_number>.pdf into output/letters

• Testing

  • Moved ENV_FILE = ".env.test" to the top of tests/conftest.py so our test DB URL is picked up before importing config.settings
  • Added tests/test_generate_welcome_letter_route.py covering:
    • Successful PDF creation
    • “Member not found” (404)
    • “No memberships” (400)

• Manually exercise the new route

    curl -X POST http://localhost:8000/members/members/1/generate_welcome_letter

• You should see output/letters/welcome_letter_.pdf appear.