Creates POST /api/familyguard-flyer endpoint that validates email format
and DSGVO consent, records download intent in Supabase flyer_downloads table,
and sends notification and download link emails (fire-and-forget) before
returning the flyer download URL.
Implements Task 4 of 7-task email-gated flyer download feature.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LhY1QhDXGWfyhrxaJvfpNt
