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/docker-compose.yml

@@ -28,6 +28,12 @@ services:
       ICS_TOKEN: ${ICS_TOKEN}
       SHEET_ID: ${SHEET_ID}
       GOOGLE_SA_JSON: /run/secrets/gcp-sa.json
+      SMTP_HOST: ${SMTP_HOST}
+      SMTP_PORT: ${SMTP_PORT}
+      SMTP_USERNAME: ${SMTP_USERNAME}
+      SMTP_PASSWORD: ${SMTP_PASSWORD}
+      MAIL_FALLBACK_FROM: ${MAIL_FALLBACK_FROM}
+      PUBLIC_BASE_URL: ${PUBLIC_BASE_URL}
     volumes:
       - ./secrets/gcp-sa.json:/run/secrets/gcp-sa.json:ro
     depends_on: