When an admin creates an invitation with an email address, an email is
now automatically sent containing the registration link and an attached
PDF guide (3 pages: registration steps, feature overview, contact info).
- Add fpdf2 for PDF generation with Unicode font support
- Add PDF guide generator (backend/app/services/pdf_guide.py)
- Extend send_email() to support file attachments
- Fire-and-forget email in create_invitation endpoint
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
