Add event receiver framework for webhook-driven graph updates

Bắt đầu với docs/docs/roadmap.md và pipeline meteor run và trích xuất hiện có; issue không xác định các tệp triển khai hoặc test. Công việc hoàn tất khi việc nhận webhook, các handler có thể cắm, hỗ trợ nguồn tích hợp sẵn, việc xác thực và tích hợp với pipeline đồ thị được định nghĩa và được bao phủ bởi các test.

Context

Not every metadata change needs a full extraction cycle. Many sources can push notifications — a GitHub webhook on a schema migration merge, a PagerDuty alert when an incident opens, a cloud event when a resource is created. Meteor should be able to receive these events and convert them into graph updates.

Scope

• Lightweight HTTP server for receiving webhooks (meteor receive or built into meteor run)
• Pluggable event handler interface — map incoming events to asset/relationship mutations
• Built-in handlers for common sources:
  • GitHub webhooks (push, PR merge, release)
  • Generic cloud events (GCP Pub/Sub, AWS EventBridge)
• Feed received events into the same processing pipeline as extracted records
• Authentication and validation for incoming webhooks

Design Considerations

• Event receivers complement extractors, they don't replace them. Full extraction provides the baseline; events provide real-time updates.
• Should support running alongside batch extraction in the same Meteor process
• Events should carry enough metadata to route to the right graph update

Why

Batch extraction on a 1-hour schedule means the graph can be up to 1 hour stale. Event receivers close this gap for sources that support push notifications, making the graph near-real-time for critical changes.

References

• Meteor Roadmap — Event Receivers