perf: eliminate N+1 Stripe lookups in /products endpoint

[vincerevu]

Summary

This PR improves /products performance by removing the N+1 Stripe lookup pattern. Instead of making one Stripe request per product to resolve Stripe-specific IDs, it fetches Stripe products once up front and maps them locally using metadata.product_id.

What changed

• replaced per-product Stripe lookups with a single upfront stripe.products.list call
• built a local map from metadata.product_id to Stripe product IDs
• updated product retrieval logic to use local lookups during iteration
• added benchmark coverage for the optimized /products flow
• included related benchmark package / lock / ignore file updates

Why

Previously, the retrieval loop made one Stripe request for each configured product, so latency scaled linearly with the size of PRODUCT_CONFIG. This introduced unnecessary network overhead and slower endpoint performance.

Measured improvement

The benchmark simulates 5 serial calls to /products.

• before the patch: 15 calls to products.list, completed in 758 ms
• after the patch: 5 calls to products.list, with lower total request overhead

Impact

• removes the N+1 API call pattern
• reduces repeated Stripe network I/O
• improves response time and scalability for product retrieval

Verification

• added benchmark logic to validate the new behavior
• confirmed the endpoint now uses one upfront Stripe fetch and local mapping afterward
• reviewed the related benchmark environment file changes included in the patch