feature/1043: Integrate and configure SlowAPI rate limiting in the application

[babakjahan]

We need to integrate SlowAPI to enable request rate limiting across API endpoints.
This will help protect the application from abuse, prevent API flooding, and improve service reliability.

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✅ Tasks

• Install SlowAPI package
• Initialize Limiter instance in FastAPI application startup
• Configure key_func using get_remote_address
• Set up default in-memory backend for development
• Add Redis backend configuration for production environments
• Apply rate limits on key API routes (e.g., /auth/login, /cart/*, /products)
• Add error handler for RateLimitExceeded
• Add integration test to verify rate-limiting works
• Update API documentation with rate-limit information

💡 Notes

• For local testing, the in-memory backend is sufficient.
• In production, Redis must be shared across all instances for consistent limits.
• Reference: SlowAPI GitHub