feat: decouple email delivery with application events, asynchronous execution, and Resilience4j retries

[podlLev]

Overview

This PR refactors the notification delivery layer by decoupling mail operations from HTTP request threads using Spring Application Events and asynchronous processing. It also adds Resilience4j fault tolerance to handle transient SMTP delivery failures without blocking client workflows.

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Key Changes

• Event-Driven Architecture & Decoupling:

  • Application Events: Introduced VerificationEmailRequestedEvent and PasswordResetEmailRequestedEvent domain events to represent mail requests cleanly.
  • Event Publisher: Refactored business services to dispatch events rather than calling MailService directly, eliminating tight coupling.

• Asynchronous Execution & Resilience:

  • Transaction-Aware Async Processing: Created MailEventListener to handle mail dispatch asynchronously post-transaction commit (@TransactionalEventListener), preventing email delivery latency from delaying HTTP responses.
  • ThreadPool Configuration: Configured a dedicated mailTaskExecutor bean and enabled @Async support for isolated concurrency management.
  • SMTP Fault Tolerance: Integrated Resilience4j retry mechanisms into the mail delivery pipeline to automatically recover from transient SMTP errors.

• Test Suite Updates:

  • Unit & Integration Tests: Updated MailServiceImplTest and VerificationServiceImplTest to reflect event publication logic and added MailEventListenerTest to cover event listener behavior and retries.