[registration] add returning member lookup by UPI and student ID

[WilliamTayNZ]

Important: When you begin this ticket, create a new branch from feat/registration-rework and not main. When you open a PR, open it into feat/registration-rework and not main.

Description

The new registration flow replaces the current self-report "have you registered previously" question with an actual database lookup. After a UoA student provides their UPI and student ID, the system checks whether they were a member in the previous year. If found, their existing details (email, first name, last name, faculty, programme, major) are displayed, and the user is allowed to change any details if they like. This ticket covers the backend lookup function only - integrating it into the form flow is a separate ticket.

Requirements

• Add a lookup function to the member repository (e.g. findMemberByUpiAndStudentId) that queries the Member table
• Return the member's existing data if found, null if not
• The function should be pure query - no side effects, no writes, no validation beyond "does this row exist"

Acceptance Criteria

• Function exists in the member repository and is exported
• Returns the full member record (or a defined subset of fields) when a matching UPI + studentId combination exists
• Returns null when no match is found

Notes

This works against the existing schema - the new fields from ticket #83 don't affect whether a lookup works, only what data it returns. The function's return type may need updating once ticket #83's schema changes land, but the query and test structure won't change.