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/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