Harden Xero webhook signature validation

[MAJINSI]

Summary

• move the Xero webhook signing key out of controller code and require XERO_WEBHOOK_SIGNING_KEY at runtime
• validate the signature over request->rawBody, matching webhook HMAC expectations
• compare signatures with hash_equals() and fail closed with framework 401 responses instead of echo/die output
• add docs and a static regression guard for this webhook path

/claim #55

Verification

• tests/check-xero-webhook-signature-hardening.sh
• git diff --check
• php -l admin/modules/v1/controllers/XeroWebhookController.php could not be run locally because this environment does not have php installed

Scope

This is a narrow Xero webhook signing-key validation slice. It does not touch Xero OAuth client-secret config already covered by other PRs, AWS/IAM/S3, Civil ID flows, OneSignal, service tokens, live Xero access, or real private signing-key values.

Transparency: AI-assisted with OpenAI Codex; I reviewed the diff and verification before submitting.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e7b4326e-6387-4954-9be3-855892ed85b5

📥 Commits

Reviewing files that changed from the base of the PR and between 7b023ff and 2d3e306.

📒 Files selected for processing (3)

• admin/modules/v1/controllers/XeroWebhookController.php
• docs/xero-webhook-signing-key.md
• tests/check-xero-webhook-signature-hardening.sh

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[MAJINSI]

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