Add bounded async event processing with worker-pool backpressure controls

[darthfork]

Parent issue: #35

Summary

The webhook handler currently performs validation, parsing, and metric updates synchronously before acknowledging the request. Introduce a bounded asynchronous processing pipeline so acknowledgements stay fast under bursty traffic.

Deliverables

• Internal queue for validated events.
• Worker pool with configurable concurrency.
• Metrics for queue depth, dropped events, processing latency, and failures.
• Clear overload behavior when queue is full.

Acceptance criteria

• Webhook ack path remains fast under load.
• Processing throughput and backpressure are visible via metrics.
• Failure modes are documented.