Booking confirmation email + customer self-service cancel/reschedule (#18)

Sends a confirmation email (best-effort, fire-and-forget SMTP via mailer.py)
on booking creation, with a manage-booking link embedding the ticket-2 signed
token. Adds /manage/<token>, a stateless cancel/reschedule page that reuses
the existing slot-picker against booking_api's create/cancel/reschedule API,
distinguishing an invalid/expired link from an already-cancelled one. Sender
address uses the client's own domain when configured, falling back to a
mivanchenko.de address otherwise.

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

backoffice/.env.example

@@ -3,3 +3,13 @@ DB_PASSWORD=change-me-strong
 CRM_API_TOKEN=change-me-long-random
 BOOKING_TOKEN_SECRET=change-me-long-random-too
 SHEET_ID=1raMSWRZw_JfHlWqOb3LbhaQ6LWx0VGblxIV4Z2pSzp8
+# Booking confirmation email (#18). Left blank, sending is skipped (logged,
+# not fatal) -- mail relay setup is a separate infra/triage item.
+SMTP_HOST=
+SMTP_PORT=587
+SMTP_USERNAME=
+SMTP_PASSWORD=
+# Used as the From address when a client has no domain configured.
+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