Add deployment queue limit to prevent queue bombing

- Add configurable deployment_queue_limit server setting (default: 25)
- Check queue size before accepting new deployments
- Return 429 status for webhooks/API when queue is full (allows retry)
- Show error toast in UI when queue limit reached
- Add UI control in Server Advanced settings

Fixes #6708

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Models/ServerSetting.php

@@ -13,6 +13,7 @@ use OpenApi\Attributes as OA;
     properties: [
         'id' => ['type' => 'integer'],
         'concurrent_builds' => ['type' => 'integer'],
+        'deployment_queue_limit' => ['type' => 'integer'],
         'dynamic_timeout' => ['type' => 'integer'],
         'force_disabled' => ['type' => 'boolean'],
         'force_server_cleanup' => ['type' => 'boolean'],