test: cover marketplace preflight featured and stats routes

[jakerated-r]

Summary

• add preflight route coverage for eligible buyers, ineligible buyer/sold-out results, missing buyerAddress, and malformed buyerAddress
• add featured marketplace route coverage for populated and empty responses, cache/security headers, and rate limiting
• add marketplace stats route coverage for cache hit, cache miss, zero-listing stats, and rate limiting

Closes #595

Validation

• npm ci
• npm test -- --run tests/api/marketplace-preflight.test.ts tests/api/marketplace-featured.test.ts tests/api/marketplace-stats.test.ts tests/api/marketplace-purchase.test.ts
• git diff --cached --check

Note: the targeted run passes and emits an existing Vite warning for the duplicate marketplaceStats key in src/lib/backend/cache/index.ts; this patch does not modify that module.

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