BUY-65260 add stale search read cache fallback

[BuyWhere]

Summary

• add 24h stale Redis shadow entries for /v1/products/search responses
• add normalized warm-query pointers for unfiltered head searches so replay variants can reuse a warm/stale response
• count exact/stale responses as cache hits and expose X-Cache: STALE for fallback hits

Verification

• npm run build
• node --test --test-force-exit --test-name-pattern 'checks Redis cache before querying DB|stores result in Redis after DB query' tests/search.test.mjs tests/cache-stats.test.mjs
• live pre-deploy probe (current prod) still shows cold multi-word searches at 6.76-10.11s, confirming the issue this PR targets before deployment

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