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/Livewire/Project/Application/Previews.php

@@ -249,6 +249,11 @@ class Previews extends Component
                 pull_request_id: $pull_request_id,
                 git_type: $found->git_type ?? null,
             );
+            if ($result['status'] === 'queue_full') {
+                $this->dispatch('error', 'Deployment queue full', $result['message']);
+
+                return;
+            }
             if ($result['status'] === 'skipped') {
                 $this->dispatch('success', 'Deployment skipped', $result['message']);