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/Heading.php

@@ -100,6 +100,11 @@ class Heading extends Component
             deployment_uuid: $this->deploymentUuid,
             force_rebuild: $force_rebuild,
         );
+        if ($result['status'] === 'queue_full') {
+            $this->dispatch('error', 'Deployment queue full', $result['message']);
+
+            return;
+        }
         if ($result['status'] === 'skipped') {
             $this->dispatch('error', 'Deployment skipped', $result['message']);
 
@@ -151,6 +156,11 @@ class Heading extends Component
             deployment_uuid: $this->deploymentUuid,
             restart_only: true,
         );
+        if ($result['status'] === 'queue_full') {
+            $this->dispatch('error', 'Deployment queue full', $result['message']);
+
+            return;
+        }
         if ($result['status'] === 'skipped') {
             $this->dispatch('success', 'Deployment skipped', $result['message']);