warn: log when in-memory rate limiter used in production (closes #2412)

[Rudrasamadhiya]

Summary

When UPSTASH_REDIS_REST_URL is not configured in production, the middleware silently falls back to an in-memory rate limiter (memoryBuckets). Because serverless environments create new instances on cold starts, the in-memory counter resets to zero on every cold start, making rate limiting ineffective.

This PR adds a console.warn at module initialization time that fires in production when the Upstash credentials are absent, alerting operators that they are running without persistent rate limiting.

Closes #2412

Type of Change

• Bug fix / observability improvement

[github-actions]

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[github-actions]

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