feat: add webhook retry monitoring summary

[yuliuyi717-ux]

/claim #130

Summary

• add a delivery monitoring summary endpoint for webhook retries
• expose webhook delivery lifecycle metrics for retry/success/failure visibility
• document the new monitoring surface in the README and OpenAPI spec

What changed

• added GET /webhooks/deliveries/summary with backlog and failure timestamps
• tracked webhook delivery events in Prometheus via finmind_webhook_delivery_events_total
• added focused backend tests for summary aggregation and retry metrics

Validation

• REDIS_URL=redis://localhost:6379/15 PYTHONPATH=. ../../.venv/bin/pytest tests/test_webhooks.py::test_webhook_delivery_summary_reports_retry_backlog tests/test_observability.py::test_metrics_endpoint_exposes_webhook_retry_metrics -q
• python3 -m py_compile packages/backend/app/observability.py packages/backend/app/routes/webhooks.py packages/backend/app/services/webhooks.py

Demo

• recorded a short sanitized terminal demo covering the new endpoint, metrics, and passing targeted tests

[yuliuyi717-ux]

finmind_130_demo.mp4