feat: implement centralized rate limiters and structured 429 response handler

[Babin123456]

Closes #2549

Summary

Implements multi-tiered, environment-configurable rate limiters for authentication and order endpoints, featuring structured JSON outputs for HTTP 429 exceptions to allow cleaner frontend error display.

Changes Made

• Rate Limit Config: Rewrote app/limiter.py to create auth_limiter, order_limiter, wishlist_limiter and custom exception handlers.
• Main Registration: Updated app/main.py to register rate_limit_exceeded_handler to capture RateLimitExceeded exceptions.
• Endpoint Protection: Applied limit decorators on /api/auth/register, /api/auth/login, and /api/orders/ endpoints.
• Tests: Created tests/test_rate_limits.py to verify that hitting limits yields a structured 429 error format.

Verification Plan

Automated Tests

• Run: pytest backend/tests/test_rate_limits.py

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[Babin123456]

@janavipandole Check it once...

[janavipandole]

rise new on pls