Restore account balance when deleting transactions - #1927

Summary

• Reverse a transaction's effect on the account balance when it is deleted.
• Export the Express app without starting the listener when it is imported for testing.
• Add an end-to-end regression test and an npm test command for the bank API.

Problem

Creating a transaction updates account.balance, but deleting the same transaction previously removed only the transaction entry. The API could therefore return a transaction history that no longer reconciled with its reported balance.

For example, starting at 75, adding a +10 transaction produced 85. Deleting that transaction returned HTTP 204 and removed it from the ledger, but the balance remained 85.

Testing

npm test
npm run lint -- server.js server.test.js

Both commands pass against the published branch contents.