fix: Implement IP-based rate limiting with Redis for DDoS protection

[Darktan242]

Summary

Implemented IP-based rate limiting with Redis for DDoS protection in the stellar-payment-platform.

Changes made:

1. server.js: Added a strict authLimiter for /auth endpoints (email OTP verification / login flow). This limiter:

   • Uses the same Redis-backed RedisStore as the global limiter, so limits are shared across all distributed Node instances.
   • Applies a much tighter budget (max 20 requests per 15-minute window) compared to the global limiter (max 100), protecting against brute-force attacks on auth/login endpoints.
   • Keys by client IP, which is appropriate for unauthenticated auth endpoints.
   • Returns the same standardized RATE_LIMITED error body and standard/legacy rate-limit headers.

2. tests/rate-limit.test.js: Added a test verifying that the strict auth limiter rejects the 21st request to /auth/verify-email (429) even though the global limit is 100.

The global limiter already used RedisStore when a Redis client is available (existing code), satisfying the "Redis store configured for rate limiting" and "Global limits applied correctly across distributed nodes" acceptance criteria. The new auth limiter satisfies the "Specific strict limits for auth/login endpoints" criterion.

Changed files

• stellar-payment-platform/server.js
• stellar-payment-platform/tests/rate-limit.test.js

Test plan

Run the rate-limit test suite: cd stellar-payment-platform && npm test -- --testPathPattern=rate-limit (or npm test for the full suite). The new test verifies the strict auth limiter returns 429 on the 21st request to /auth/verify-email. Existing tests verify the global limiter still works with the Redis-backed store.

This PR was created as a draft by the issue solver bot. It will remain a
draft until repository CI passes.

Closes #492

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