Feat: Add Admin Dashboard

[Raindancer118]

This pull request introduces a comprehensive set of changes to support admin functionality in the backend. It adds an is_admin field to the User model, implements admin-only API endpoints for user management, and enforces admin access via middleware. The changes include new handlers for admin operations, route setup, and logic to seed an initial admin user.

Admin API and Access Control:

• Added AdminHandler in backend/handlers/admin.go with endpoints for dashboard stats, user listing/search, user promotion to admin, user deletion, verification, update, and password reset. All endpoints require admin authentication and password confirmation for sensitive actions.
• Introduced AdminMiddleware in backend/middleware/auth.go to restrict access to admin routes, ensuring only authenticated admin users can access these endpoints.
• Registered new admin routes in backend/main.go under /v1/admin, applying both authentication and admin middleware. [1] [2]

User Model and API Enhancements:

• Added IsAdmin boolean field to the User model in backend/models/models.go, and exposed it in the user API responses. [1] [2] [3]

Admin User Initialization:

• Added a seedAdmin function in backend/main.go to create or update an initial admin user (nora.admin@nordakademie.de) with a default password and admin privileges.

Other Improvements:

• Updated imports in backend/main.go to support new functionality (e.g., uuid, utils, models). [1] [2]
• Added comments and error handling throughout the new admin logic for clarity and robustness.

These changes collectively provide a secure and robust foundation for admin operations in the application.

[FinnK04]

In the backend, create a new folder containing all backend files so that they correspond to the new planned structure.

[FinnK04]

Please add the changes you made to your .env file to the .env.example file.
Then rebase your changes, squash them, and give them a meaningful name :)