feat: tick planner, delivery job, scheduler and purge

Also: test uses disabled channels instead of deleting them (FK cascade); ChannelRegistry no longer final so tests can extend it (controller ruling).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BCrwHHnGCB5XH968Nxokqw

core/app/Delivery/DeliveryFailed.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace App\Delivery;
+
+use RuntimeException;
+
+final class DeliveryFailed extends RuntimeException {}