Owner notification webhook: Telegram re-point (#23)

Flask fires a fire-and-forget internal webhook (owner_notify.py, mirroring
mailer.py's background-thread queue) on booking create/cancel/reschedule,
carrying the same {booking, business_name, notify_channel} shape the old
EA-driven "Build booking row" node produced, plus an event field so the
Telegram message can say what actually happened. n8n/booking-sync.json gets
a new webhook + IF node feeding the existing Telegram node directly, so it
no longer needs EA's API to build the notification payload; channel routing
(only telegram sends for now) lives in that IF node.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/.env.example

@@ -15,3 +15,8 @@ SMTP_PASSWORD=
 MAIL_FALLBACK_FROM=noreply@mivanchenko.de
 # Base URL the manage-booking link in the confirmation email is built from.
 PUBLIC_BASE_URL=https://onboard.mivanchenko.de
+# Owner Telegram notification (#23). Points at n8n's "Owner notify webhook"
+# node (n8n/booking-sync.json), e.g. https://n8n.mivanchenko.de/webhook/owner-notify.
+# Left blank, sending is skipped (logged, not fatal) -- same convention as
+# SMTP_HOST above.
+OWNER_NOTIFY_WEBHOOK_URL=