BE: Initialize Swagger API Documentation

## 📚 Overview

Implement Swagger API documentation in the BudgetChain Backend project to provide clear and accessible API definitions and usage guidelines.

## 🛠️ Task Details

- **Configure Swagger/OpenAPI:**
- Integrate and configure the `@nestjs/swagger` module with existing NestJS framework setup.

- **Set Up API Documentation Structure:**
- Organize the API documentation structure to ensure it is scalable and maintainable.
- Ensure that the documentation dynamically reflects all available endpoints and modules (such as auth, treasury, budget, ai, blockchain, reporting, and user management).

- **Create Initial API Documentation:**
- Document the core endpoints and provide basic usage examples.
- Validate the configuration against both development and production environments.

## ⚙️ Technical Considerations

- Make sure to follow existing project dependencies and coding standards as outlined in [BudgetChain Backend README](https://github.com/BudgetChain/BudgetChain-Backend?tab=readme-ov-file).
- Ensure the configuration seamlessly integrates with the current NestJS setup.
- Provide clear instructions on how to update and maintain the API documentation.

## 🎯 Definition of Done

1. Swagger/OpenAPI is integrated and configured correctly in the project.
2. The API documentation is accessible and auto-generated based on existing endpoints.
3. Documentation reflects initial core endpoints with usage instructions.
4. Clear setup instructions are added for future maintenance.

Let's get this set up to ensure a better development experience and easier integration for third party developers! 🚀

Start by reading the BudgetChain Backend README and the existing NestJS setup to find the application entry point and current dependencies. Review how auth, treasury, budget, AI, blockchain, reporting, and user-management endpoints are organized. Done means Swagger/OpenAPI is accessible, reflects the available endpoints in development and production, includes core usage guidance, and has maintenance instructions.

Written by the indexing model from the issue text.