Implement Debt & Plan Entities

[krschan]

Create the Debt and Plan entities to manage financial tracking.

Tasks:

1. Create Debt Entity

   • Define Debt.java with fields: id, amount, userId, groupId, dueDate.
   • Implement DebtRepository.java for database operations.

2. Create Plan Entity

   • Define Plan.java with fields: id, name, details.
   • Implement PlanRepository.java for database operations.

3. Update Entity Relationships

   • Ensure debts and plans are correctly linked to users and groups.

4. Test Debt and Plan Entities

   • Verify that database persistence works correctly.
   • Write unit tests for DebtRepository.java and PlanRepository.java.