feat: add signed webhook delivery pipeline

[wzh886]

Summary

• add outbound webhook subscription CRUD endpoints and signed delivery pipeline
• add webhook queue processing, retries, metrics, and worker entrypoint
• document webhook API and delivery semantics in README and OpenAPI
• preserve reminder/bill event behavior with targeted regression coverage

Validation

• python -m pytest -q tests/test_webhooks.py::test_reminder_sent_event_emitted_only_on_successful_send -vv
• python -m pytest -q tests/test_bills.py -x
• python -m pytest -q tests/test_reminders.py -x
• python -m pytest -q tests/test_observability.py -x

Notes

• non-blocking deprecation warnings remain around datetime.utcnow(); functionality is validated but time handling should be modernized later to timezone-aware UTC.