[RELIABILITY][AUTH AUDIT] Make authentication-event persistence observable and durable

[Obiajulu-gif]

Summary

Authentication events are written fire-and-forget and every database error is swallowed. Incident-response history can silently disappear even though the request path logs success.

Repository evidence

• lib/auth/auth-event-log.ts:82-103 documents and implements an unawaited AuthEventModel.create(...).catch(() => {}).
• lib/auth/auth-event-log.ts:114-116 relies on that collection for incident review.

Scope

• Queue auth events through a durable bounded path or await critical event classes.
• Emit metrics/alerts for dropped events and validate field-size limits before logging.

Acceptance criteria

• Critical auth events are not acknowledged without durable acceptance.
• Dropped noncritical events are counted with redacted diagnostics.

Tests

• Inject database outages, queue saturation, restart, duplicate delivery, and oversized headers.

Non-goals

• Replacing the broader financial audit trail.

[Owolabenjade]

Hello maintainer, I want to work on this issue.

[grantfox-oss]

🦊 GrantFox — @Owolabenjade has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #193)
2. Your PR will be reviewed by the Chainmove maintainers

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