add IP-based rate limiting to Cloudflare Worker

[hemanth5055]

Description:
This PR introduces IP-based rate limiting to the Cloudflare Worker handling /v1/translate and /v1/explain routes. The main goal is to protect the Gemini API from abuse and prevent excessive requests that could exhaust our quota.

Changes Made:

• Implemented checkRateLimit function using Cloudflare KV to track request counts per IP.
• Set a limit of 10 requests per minute per IP, returning a 429 Too Many Requests error if exceeded.
• Updated fetch handler to integrate the rate limiting check before processing requests.
• Added JSON parse safety and TTL management for KV storage.
• Retained existing functionality for /v1/translate and /v1/explain routes.
• Added a new test route /test-rate-limit to verify rate limiting logic

Benefits:

• Prevents API abuse and uncontrolled usage.
• Protects the Gemini API quota and reduces potential costs.
• Improves overall security and stability of the service.

Screenshots:

[dineshsutihar]

It looks good from a quick glance. Let me pull it down to my system and verify everything, and then we should be good to merge.

[Copilot]

Pull Request Overview

This PR adds IP-based rate limiting to protect the Cloudflare Worker's /v1/translate and /v1/explain routes from abuse. The implementation sets a limit of 10 requests per minute per IP address using Cloudflare KV storage.

• Introduces a checkRateLimit function that tracks request counts per IP using KV storage
• Integrates rate limiting into the main request handler with 429 responses for exceeded limits
• Adds KV namespace configuration and a test endpoint for verification

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 4 comments.

File	Description
backend/wrangler.jsonc	Adds KV namespace binding for rate limiting storage
backend/src/index.ts	Implements rate limiting logic and integrates it into the request handler

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[dineshsutihar]

LGTM, Good Work @hemanth5055