Add review notes for Multiple Email Support MR

[Lan2u]

Motivation

• Provide a focused code review for the "Multiple Email Support" MR that captures code-quality issues, technical debt and security/privacy concerns discovered while inspecting the related authentication and read-model changes.

Description

• Add reviews/multiple-email-support-review.md describing three findings and actionable recommendations: (1) case-sensitive uniqueness checks in link-number-to-email can create ambiguous identities relative to login-time normalisation, (2) missing test coverage for the members.length > 1 branch in sendLogInLink, and (3) PII exposure via logging the submitted email in ambiguous-member error handling.

Testing

• No automated tests were run because this is a documentation-only change; the commit only adds the review document and does not modify runtime code or test suites.

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Codex Task