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>

openapi.yaml

@@ -6312,6 +6312,8 @@ components:
           type: integer
         concurrent_builds:
           type: integer
+        deployment_queue_limit:
+          type: integer
         dynamic_timeout:
           type: integer
         force_disabled: