add payments functionality

[andrew2net]

This PR allows payment for a given loan to be created using a JSON API call. The following features and enhancements have been implemented:

1. Payment Creation:

• Added the ability to create payments for a loan.
• Payments store the payment date and amount.
• Ensured that a payment cannot be created if it exceeds the outstanding balance of the loan.
• Added validation errors for cases where a payment cannot be created.

2. Outstanding Balance Calculation:

• Exposed the outstanding balance for a given loan in the JSON response for the LoansController#show and LoansController#index.
• The outstanding balance is calculated as the funded_amount minus the sum of all payment amounts.

3. Endpoints for Payments:

• Added an endpoint /loans/:loan_id/payments to retrieve all payments for a given loan.
• Added an endpoint /loans/:loan_id/payments/:id to retrieve an individual payment.

4. Thread Safety: Ensured thread safety when creating payments to prevent race conditions and maintain data integrity.

Changes

• Models:

  • Loan: Added outstanding_balance method to calculate the remaining balance.
  • Created a Payment model and added validation to ensure a payment amount does not exceed the loan's outstanding balance.

• Controllers:

  • LoansController: Updated index and show actions to include outstanding_balance in the JSON response.
  • Created PaymentsController and added index, show, and create actions to handle payment retrieval and creation.

• Specs:

  • Added tests for LoansController to verify the inclusion of outstanding_balance in the response.
  • Added tests for PaymentsController to verify payment creation and retrieval.
  • Added tests for the Payment model to ensure thread safety and validation.