Add transaction model, controller, and routes with validation for expense category

[ajinkya8010]

🚀 Pull Request

📋 Description

This PR implements the backend logic for transactions, including the Mongoose model, controller functions, and Express routes. It enforces that the category field is required only for expense transactions, while allowing it to be optional for income transactions. The endpoints support creating, fetching, and deleting transactions for the authenticated user, with robust error handling and validation.

Fixes #226

🔍 Type of Change

• Bug fix 🐞
• New feature ✨
• Documentation update 📝
• Refactoring or code improvement ♻️
• Other (please describe):

🙋 Your Details

• Name: Ajinkya
• Email ID: ajinkya8975@gmail.com

🧪 How Has This Been Tested?

Locally on postman

📸 Screenshots (if applicable)

Before	After

✅ Checklist

• I have read the contributing guidelines.
• I have followed the code style and linting rules.
• I have added tests or explained why not.
• I have updated documentation (if needed).
• My changes do not introduce any known security issues or vulnerabilities.

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