feat(scheduling): persist and claim scheduled job occurrences

Add database-backed schedule states and deliveries so distributed schedulers publish each occurrence once, queue jobs claim executions atomically, and stale occurrences are cleaned up.

app/Models/ScheduledJobDelivery.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Models;
+
+class ScheduledJobDelivery extends BaseModel
+{
+    protected $fillable = [
+        'uuid',
+        'schedule_key',
+        'scheduled_for',
+        'job_type',
+        'resource_id',
+        'payload',
+        'status',
+        'claim_token',
+        'enqueued_at',
+        'started_at',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'scheduled_for' => 'immutable_datetime',
+            'payload' => 'array',
+            'enqueued_at' => 'immutable_datetime',
+            'started_at' => 'immutable_datetime',
+        ];
+    }
+}