feat(auth): limit challenge and signature abuse

[mayborn005]

Summary

• Add configurable fixed-window limits for challenge issuance by IP and wallet, plus a separate limit for failed signature attempts.
• Record sanitized audit events for issued challenges and rate-limit rejections; hashed wallet identifiers are used in auth audit records.
• Document thresholds and legitimate retry behavior, with focused regression coverage.

Closes #45

Checks

• git diff --check passed.
• Jest could not run because dependencies are not installed (jest: command not found); no install or build was run.

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[mayborn005]

@Adesokan-2 I resolved the merge conflict and updated the branch. The remaining lint failure is an unused import already present on the current base branch; could you review the PR and advise on that shared check?