feat: implement rate limiting on express server to prevent abuse

[Johnsource-hub]

#closes
#381

Done. Branch feat/rate-limit-headers is ready with all 7 tests passing.
Changes:

• stellar-payment-platform/server.js — Added express-rate-limit middleware (100 req/15 min) with legacyHeaders: true to expose X-RateLimit-Limit, X-RateLimit-Remaining, and X-RateLimit-Reset headers on every response.
• stellar-payment-platform/tests/rate-limit.test.js — 7 tests covering header presence, values, decrement behavior, 429 responses, and header persistence on 429s.

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[Abdulazeem-code]

Please resolve conflicts

[Abdulazeem-code]

please resolve all the conflicts