Fix PHPStan level 6 errors from CI

- Remove unused AvailabilityRepository/BookingRepository from BookingPage
  and ShortcodeRegistrar (template is a JS shell; no PHP data needed yet)
- Add @param array<string, string> to shortcode render() signatures
- Add @return array<string, mixed> to model toArray() methods
- Fix get_permalink() ?? '' — returns string|false not nullable, use cast
- Remove unused ignoreErrors pattern from phpstan.neon

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

phpstan.neon

@@ -7,5 +7,3 @@ parameters:
         - src
     bootstrapFiles:
         - tests/bootstrap.php
-    ignoreErrors:
-        - '#Unsafe usage of new static\(\)#'