feat: database-backed cron job queue replacing JSON file system

Replace file-based JSON cron queue with DB-backed job queue (pp_cron_jobs,
pp_cron_schedules). New Domain\CronJob module: CronJobType (constants),
CronJobRepository (CRUD, atomic fetch, retry/backoff), CronJobProcessor
(orchestration with handler registration). Priority ordering guarantees
apilo_send_order (40) runs before sync tasks (50). Includes cron.php auth
protection, race condition fix in fetchNext, API response validation,
and DI wiring across all entry points. 41 new tests.

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

autoload/admin/App.php

@@ -423,7 +423,8 @@ class App
             new \Domain\Order\OrderRepository( $mdb ),
             $productRepo,
             new \Domain\Settings\SettingsRepository( $mdb ),
-            new \Domain\Transport\TransportRepository( $mdb )
+            new \Domain\Transport\TransportRepository( $mdb ),
+            new \Domain\CronJob\CronJobRepository( $mdb )
           ),
           $productRepo
         );