feat: due resolution and cascade math

Also: StoresUtc trait normalizes datetime casts to UTC (controller ruling).

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

core/app/Models/Concerns/StoresUtc.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Models\Concerns;
+
+use Carbon\CarbonImmutable;
+use DateTimeInterface;
+
+/**
+ * Eloquent форматирует datetime без смещения, поэтому Carbon в чужом поясе
+ * записался бы как «те же цифры в UTC». Приводим к UTC при присвоении.
+ */
+trait StoresUtc
+{
+    public function setAttribute($key, $value)
+    {
+        if ($value instanceof DateTimeInterface
+            && in_array($this->getCasts()[$key] ?? null, ['datetime', 'immutable_datetime'], true)) {
+            $value = CarbonImmutable::instance($value)->utc();
+        }
+
+        return parent::setAttribute($key, $value);
+    }
+}

core/app/Models/Delivery.php

@@ -3,11 +3,14 @@
 namespace App\Models;
 
 use App\Enums\DeliveryResult;
+use App\Models\Concerns\StoresUtc;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
 class Delivery extends Model
 {
+    use StoresUtc;
+
     protected $guarded = [];
 
     protected $attributes = ['meta' => '{}'];

core/app/Models/Event.php

@@ -5,12 +5,15 @@ namespace App\Models;
 use App\Enums\AfterDue;
 use App\Enums\DueMode;
 use App\Enums\EventState;
+use App\Models\Concerns\StoresUtc;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 
 class Event extends Model
 {
+    use StoresUtc;
+
     protected $guarded = [];
 
     protected function casts(): array