Database fixtures

[shadrach68]

closes #396

Summary

Test fixtures using randomized data can produce non-deterministic results that are hard to debug when tests fail. This PR introduces a new fixture helper that generates stable, deterministic creator records from a seed value, making failures reproducible and easier to investigate.

Changes

• New Helper: Added createSeededCreatorFixture to generate stable test database fixtures.
• Documentation: Documented the exact mapping from seed to generated field values via JSDoc in the helper.
• Helper Tests: Added comprehensive unit tests to ensure that the same seed produces the same record, and different seeds produce distinct records.
• Test Refactoring: Refactored inline and randomized fixtures in creator.service.test.ts and creator-list-item.mapper.test.ts to use the new seeded helper.

Acceptance Criteria

• The same seed value always produces the same fixture record
• Different seed values produce distinct records
• At least two existing tests are updated to use the helper

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