Require bookings to be at least 5 days ahead

Guests could previously book for the same day or tomorrow. The earliest
selectable date is now today + 5 days, configured in one place via the
BOOKING_MIN_LEAD_DAYS constant.

- constants.php: new BOOKING_MIN_LEAD_DAYS define (5)
- Service_model::getAvailableTimes(): optional $minLeadDays param, returns
  no slots for dates closer than that; defaults to 0 so existing callers
  are unaffected
- Pages::ajax(): guest actions (getAvailableTimes, isTimeAvailable) pass the
  constant. The admin action getAvailableTimeOptions deliberately does not,
  so staff can still enter phone bookings for tomorrow.
- Pages::booking_process(): rejects an early booking_date POST and redirects
  back to /booking, guarding against a stale page or a crafted request
- booking-form.php: datepicker minDate/defaultDate and the hidden
  booking_date default all derive from the constant

Deployed and verified on test, dev and prod.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MyYsTAAA1kJLMLCResmgTE

application/config/constants.php

@@ -83,3 +83,15 @@ defined('EXIT_USER_INPUT')     OR define('EXIT_USER_INPUT', 7); // invalid user
 defined('EXIT_DATABASE')       OR define('EXIT_DATABASE', 8); // database error
 defined('EXIT__AUTO_MIN')      OR define('EXIT__AUTO_MIN', 9); // lowest automatically-assigned error code
 defined('EXIT__AUTO_MAX')      OR define('EXIT__AUTO_MAX', 125); // highest automatically-assigned error code
+
+/*
+|--------------------------------------------------------------------------
+| Booking lead time
+|--------------------------------------------------------------------------
+|
+| Minimum number of days ahead a guest can book a job. 5 means the earliest
+| selectable date is today + 5 days. Applies to the public booking form only
+| - the admin portal can still book any date.
+|
+*/
+defined('BOOKING_MIN_LEAD_DAYS') OR define('BOOKING_MIN_LEAD_DAYS', 5);