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>

src/Plugin.php

@@ -23,6 +23,6 @@ class Plugin
         (new RoleManager())->register();
         (new AdminMenu($availability, $bookings))->register();
         (new RestRegistrar($availability, $bookings))->register();
-        (new ShortcodeRegistrar($availability, $bookings))->register();
+        (new ShortcodeRegistrar())->register();
     }
 }