feat: implement rate limiting and health check endpoints

[Ridho894]

Description

This PR implements global rate limiting using @nestjs/throttler and adds a health check endpoint powered by @nestjs/terminus.
Rate limits are applied selectively to critical routes (e.g., authentication), and configurations (TTL, limit) are loaded via environment variables.
The /health endpoint checks basic application availability, including HTTP response and database connectivity.

Related Issue

N/A

Type of Change

• ✨ New feature

Checklist

• My code compiles without errors
• I have added necessary environment variables and documented them
• I have tested the new /health route manually
• Rate limiter configuration is loaded from environment variables
• Critical routes are protected with rate limiting
• I have reviewed the code to ensure consistency and clarity