Fix code scanning alert no. 1: Missing rate limiting

[harsh6754]

Fixes https://github.com/harsh6754/Solar-Power-Application/security/code-scanning/1

To fix the problem, we will introduce rate limiting to the Express application using the express-rate-limit package. This package allows us to set a maximum number of requests that a client can make within a specified time window. We will apply this rate limiter to the routes that perform database access to ensure they are protected against abuse.

1. Install the express-rate-limit package.
2. Import the express-rate-limit package in the server/routes/admin.routes.js file.
3. Configure a rate limiter with appropriate settings (e.g., 100 requests per 15 minutes).
4. Apply the rate limiter to the routes that perform database access.

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