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/tests/Feature/SchemaTest.php

@@ -75,6 +75,29 @@ class SchemaTest extends TestCase
         $this->assertSame(ChannelType::Web, $user->channels()->first()->type);
     }
 
+    public function test_datetimes_are_stored_as_utc(): void
+    {
+        $source = Source::create(['name' => 'docs', 'token_hash' => hash('sha256', 'x')]);
+        $user = User::create(['login' => 'nikita', 'tz' => 'Asia/Shanghai', 'quiet_start' => '22:00:00', 'quiet_end' => '09:00:00']);
+
+        $event = Event::create([
+            'source_id' => $source->id,
+            'user_id' => $user->id,
+            'source_ref' => 'person:42',
+            'topic' => 'birthday:2026',
+            'due_mode' => DueMode::Local,
+            'due_date' => '2026-08-14',
+            'due_time' => '24:00:00',
+            'fire_on' => '2026-08-14',
+            'after_due' => AfterDue::Expire,
+            'payload' => ['title' => 'ДР'],
+            'state' => EventState::Scheduled,
+            'quiet_until' => CarbonImmutable::parse('2026-09-10T10:00:00', 'Asia/Shanghai'),
+        ])->fresh();
+
+        $this->assertSame('2026-09-10T02:00:00+00:00', $event->quiet_until->utc()->toIso8601String());
+    }
+
     public function test_state_helpers(): void
     {
         $this->assertTrue(EventState::Done->isTerminal());