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.json

@@ -9816,6 +9816,9 @@
                     "concurrent_builds": {
                         "type": "integer"
                     },
+                    "deployment_queue_limit": {
+                        "type": "integer"
+                    },
                     "dynamic_timeout": {
                         "type": "integer"
                     },