Backend Framework Setup: Database Migrations, Models, and Dataset Interaction Endpoints

[elainefan331]

This PR introduces the full backend foundation for the project, including the initial server setup, database schema, Sequelize migrations, model definitions, and all user–dataset interaction features.

1. Backend Framework & Core Setup

• Added backend folder structure with server.js, configuration files, and initial setup packages.
• Added .sequelizerc and configuration files for environment-based DB settings.
• Set up package scripts for backend development and server start.

2. Database Schema & Migration Files

• Added initial migration files for: users, datasets, likes, saves, comments, and view history
• Added associations between tables.
• Updated migration configurations and fixed field definitions where needed.

3. ORM Models

• Added Sequelize models for all tables.
• Defined relationships across models.
• Ensured alignment between models and migrations.

4. Authentication & User Functionality

• Added register, login, and auth utilities.
• Added token handling, middleware, and helper services.

5. Dataset Activity Features

• Implemented a complete set of dataset-related interaction endpoints.
• Mapped existing CouchDB endpoints to new unified backend routes.

6. Update the age display in subjectcard.