feat: transactional safety, async Spring Events, and comprehensive test suite

[devin-ai-integration]

Summary

Part 1: Transactional Safety

• Added @Transactional to AccountService mutating methods: registerAccount(), deposit(), withdraw(), and transferAmount()
• Critical for transferAmount() which performs 4 DB writes — without it, a failure mid-transfer could lose money

Part 2: Async Migration via Spring Events

• Created TransactionEvent value class for decoupled event publishing
• Created TransactionEventListener with @Async @EventListener @Transactional for async transaction record creation
• Created AsyncConfig with @EnableAsync and a configured ThreadPoolTaskExecutor (4-8 threads, queue capacity 50)
• Refactored AccountService to inject ApplicationEventPublisher and publish events instead of inline Transaction saves
• Balance update logic remains synchronous and transactional; only the audit/transaction-record creation becomes async

Part 3: Test Generation (25 tests, all passing)

• Added H2 in-memory database test dependency to pom.xml
• Created application-test.properties with H2 configuration
• AccountServiceTest (11 unit tests): register success/duplicate, deposit success/correctAmount, withdraw success/insufficientFunds, transfer success/insufficientFunds/recipientNotFound, loadUserByUsername success/notFound
• BankControllerIntegrationTest (10 integration tests): dashboard auth/unauth, deposit, withdraw success/insufficientFunds, transfer success/recipientNotFound, register success/duplicate, transaction history
• TransferConcurrencyTest (1 test): concurrent transfers verify conservation of money (total balance = 2000 after parallel transfers)
• TransactionEventListenerTest (2 unit tests): event creates transaction, account not found throws exception

Review & Testing Checklist for Human

• Verify transferAmount() atomicity — confirm a simulated failure mid-transfer rolls back all changes (the @Transactional annotation ensures this)
• Verify async event processing — after a deposit/withdraw/transfer, confirm the Transaction audit record is eventually created in the DB
• Run the concurrency test multiple times to verify it consistently passes (race conditions can be non-deterministic)
• Test the full UI flow: register → login → deposit → withdraw → transfer → view transactions

Notes

• The @EnableAsync is on AsyncConfig (not BankappApplication), per the instruction to use one or the other
• The concurrency test uses 10 threads with alternating transfer directions between two accounts to stress-test transactional integrity
• All 25 tests pass locally against H2 in-memory database with create-drop DDL strategy

Link to Devin session: https://app.devin.ai/sessions/36de548fb18143e187f0480db0aee398
Requested by: @joao-cognition

---

Devin Review

Status	Commit
⚪ Not started	—

💡 Connect your GitHub account to enable automatic code reviews.

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring