[registration] add member repository unit tests

[WilliamTayNZ]

Description

createMembershipRegistration (in the member repository) converts a validated MemberRegistration into a Prisma .create() call against the Member table, handling per-path field mapping (returning member, current UoA student, non-UoA student) and translating a P2002 unique-constraint violation into a distinct duplicate error, versus a generic database error for anything else. It currently has no test coverage.

Requirements

• Mock getPrisma() entirely - no real database connection should be required to run this suite
• Cover the successful-creation path
• Cover field mapping for all three MemberRegistration variants: what gets included/omitted per branch (e.g. a returning member should not receive programme/primaryAffiliation; a current UoA student should receive faculty/programme/yearLevel; a non-UoA student should receive an empty faculty array and primaryAffiliation)
• Cover optional field pass-through (discordUsername)
• Cover error translation: a Prisma error with code P2002 → { type: "duplicate" }; any other Prisma error code → { type: "database" }; a non-Prisma error → also { type: "database" }

Acceptance Criteria

• memberRepository.test.ts exists, with getPrisma mocked, no live database required to run
• All three registration variants have a dedicated field-mapping test
• All three error scenarios (duplicate, other Prisma error, non-Prisma error) are covered
• Test file includes comments explaining the mocking pattern (vi.mock, spy call inspection via .mock.calls) for anyone unfamiliar with it

Notes

This is a self-contained unit test task with no dependency on the faculty/major redesign - the repository's field-mapping logic for faculty/programme will need a small update once that redesign lands, but the mocking setup and most of the file structure won't change.