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.

app/Notifications/Server/ServerPatchCheck.php

@@ -24,6 +24,16 @@ class ServerPatchCheck extends CustomEmailNotification
         return $notifiable->getEnabledChannels('server_patch');
     }
 
+    public function deduplicationKey(object $notifiable, string $channel): ?string
+    {
+        return "server-patch-check:server:{$this->server->uuid}:state:".hash('sha256', json_encode($this->patchData));
+    }
+
+    public function deduplicateFor(): int
+    {
+        return 86400;
+    }
+
     public function toMail($notifiable = null): MailMessage
     {
         $mail = new MailMessage;