feat: null-safe creator list envelope consistency check

[boys-cyberhub]

What changed

• Added normalizeCreatorListItems() helper in creators.serializers.ts that coerces null | undefined profiles to [] before mapping, so the items field is always an array.
• Updated serializeCreatorList() signature to accept CreatorProfile[] | null | undefined.
• Updated wrapPublicCreatorListResponse() in public-creator-list-envelope.utils.ts to coerce null | undefined items to [] at the envelope level (defence-in-depth).
• Added jest.setup.ts + wired setupFiles in jest.config.js so Jest can load modules that transitively import src/config.ts without requiring real env vars.
• Extended creator-feed-empty-filters.integration.test.ts with 3 tests asserting items is always an array when the data layer returns null or undefined.
• Fixed a pre-existing assertion bug in the same file: objectContaining({ verified: undefined }) incorrectly assumed Zod emits optional absent keys with undefined; corrected to check key absence instead.
• Fixed missing res.set mock in makeRes() which caused attachTimestampHeader to throw, silently swallowing all responses.

Why

Creator list endpoints may return null instead of an empty array in edge cases, breaking any client-side iteration that calls .map() or .forEach() on items without a null guard.

How to test

pnpm exec jest --testPathPatterns "creator-feed-empty-filters"
# → Tests: 29 passed

Closes #281