Add n8n webhook notifications for all booking events

- Extract notify() helper (fire-and-forget, never blocks flow)
- Fire booking_created after INSERT in POST /api/bookings
- Fire contract_signed after PDF stamp in POST /api/bookings/:id/sign
- Add GET /api/bookings/:id/contract to serve signed PDFs
- New env vars: N8N_WEBHOOK_URL, PHOTOGRAPHER_EMAIL, SITE_URL

n8n workflow (LisiLou Booking Notifications, ID c2oyyQyuzMuo1kLw):
  Webhook → Code (routes by event, builds email items) → Send Email (SMTP)
  Handles: booking_created (2 emails), contract_signed, payment_confirmed

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

api/.env.example

@@ -3,6 +3,8 @@ CORS_ORIGIN=http://localhost:8080
 
 # n8n webhook for email notifications
 N8N_WEBHOOK_URL=
+PHOTOGRAPHER_EMAIL=hello@lisilou.com
+SITE_URL=http://192.168.1.192:8080
 
 # Google Calendar (issue #3)
 GOOGLE_CALENDAR_ID=