feat(notifications): deduplicate repeated email alerts

Add notification-level deduplication keys and TTLs for deployment, backup,
server, container, scheduled task, SSL, token, and transactional emails.
Apply deduplication in email channels before sending rendered messages.

tests/Feature/ApplicationStoppedAfterRestartLimitTest.php

@@ -48,6 +48,24 @@ it('does not show the restart limit warning for a normal manual stop', function
     expect($html)->not->toContain('Stopped after reaching restart limit');
 });
 
+it('uses a semantic dedupe key for restart limit notifications', function () {
+    $application = applicationWithRestartState();
+    $application->forceFill([
+        'name' => 'crashy-app',
+        'uuid' => 'application-uuid',
+    ]);
+    $application->setRelation('environment', (object) [
+        'uuid' => 'environment-uuid',
+        'name' => 'production',
+        'project' => (object) ['uuid' => 'project-uuid'],
+    ]);
+
+    $notification = new RestartLimitReached($application);
+
+    expect($notification->deduplicationKey((object) ['id' => 1], 'mail'))->toBe('restart-limit-reached:application:application-uuid:count:2')
+        ->and($notification->deduplicateFor())->toBe(86400);
+});
+
 it('keeps restart tracking configurable when stopping an application', function () {
     $method = new ReflectionMethod(StopApplication::class, 'handle');
     $resetRestartCount = collect($method->getParameters())->firstWhere('name', 'resetRestartCount');

tests/Feature/NotificationDeduplicationTest.php

@@ -0,0 +1,75 @@
+<?php
+
+use App\Notifications\CustomEmailNotification;
+use App\Services\NotificationDeduplicator;
+use Illuminate\Notifications\Messages\MailMessage;
+use Illuminate\Support\Facades\Cache;
+
+class DedupeTestNotification extends CustomEmailNotification
+{
+    public function __construct(
+        public ?string $semanticKey = null,
+        public bool $deduplicate = true,
+        public int $ttl = 900,
+    ) {}
+
+    public function toMail(): MailMessage
+    {
+        return (new MailMessage)->subject('Test');
+    }
+
+    public function shouldDeduplicate(): bool
+    {
+        return $this->deduplicate;
+    }
+
+    public function deduplicateFor(): int
+    {
+        return $this->ttl;
+    }
+
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return $this->semanticKey;
+    }
+}
+
+beforeEach(function () {
+    Cache::flush();
+    $this->deduplicator = app(NotificationDeduplicator::class);
+    $this->notifiable = new class
+    {
+        public int $id = 123;
+    };
+});
+
+it('allows only the first identical notification fingerprint during the ttl', function () {
+    $notification = new DedupeTestNotification;
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeFalse();
+});
+
+it('allows different recipients and content through the default fingerprint', function () {
+    $notification = new DedupeTestNotification;
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['second@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Other subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Other body</p>'))->toBeTrue();
+});
+
+it('uses semantic keys instead of rendered content when provided', function () {
+    $notification = new DedupeTestNotification(semanticKey: 'event:123');
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Other subject', '<p>Other body</p>'))->toBeFalse()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['second@example.com'], 'Other subject', '<p>Other body</p>'))->toBeTrue();
+});
+
+it('allows notifications to opt out of deduplication', function () {
+    $notification = new DedupeTestNotification(deduplicate: false);
+
+    expect($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue()
+        ->and($this->deduplicator->shouldSend($this->notifiable, $notification, 'mail', ['first@example.com'], 'Subject', '<p>Body</p>'))->toBeTrue();
+});