⚡ Optimize ProductStripeService variant updates

[AJFrio]

💡 What:
Optimized ProductStripeService.updateProductVariants to parallelize the variant update loops. Previously, the code iterated over variants sequentially, awaiting Stripe API calls for each variant. The optimization replaces the for...of loops with Promise.all(variants.map(...)) to execute these operations concurrently.

🎯 Why:
Updating products with many variants was slow because each variant's price update (creation and archival) involved sequential network calls to Stripe. This created a linear dependency on the number of variants, leading to poor performance for products with multiple variants.

📊 Measured Improvement:
A benchmark was created (benchmarks/product-stripe-benchmark.js) simulating updates for 10 variants (mocking 50ms latency per Stripe call).

• Baseline (Sequential): ~1011 ms
• Optimized (Parallel): ~102 ms
• Speedup: ~10x (for 10 variants)

The total time is now roughly constant (limited by the slowest single variant update) rather than the sum of all updates.

Verified that existing tests pass and the logic for handling custom prices and archives remains correct.

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PR created automatically by Jules for task 7760807679031339881 started by @AJFrio

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