Add Cantina audit report for PR #90 (rounding and billing fix)

[RuoHan-Chen]

Summary

Adds the Cantina audit report covering the rounding and billing fix from #90 (the fixed-fee feeAmount change), and lists it in the security audit tables alongside the existing reports.

• File: audits/report-cli-cantina-eb1cc8bc-577c-422d-ad76-db7495f29a5a-2026-07-22-coinbase-commerce-payments-pr-90.pdf
• Auditor: Cantina (Spearbit)
• Date: 2026-07-22
• Scope: PR Rounding and billing fix #90 — rounding and billing fix

Changes

• Add PR Rounding and billing fix #90 Cantina audit report PDF to audits/
• Add a row to the Security Audits table in README.md
• Add the matching row to the audit table in docs/Security.md

This completes the audit provenance for the feeAmount change: the prior committed reports predate #90, and this report is the review of that specific change.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[amiecorso]

only feedback is maybe let's call this "rounding and billing change, v1.1.0" because "fix" implies something was broken when really it was just that we had need for a different product requirement around rounding