Fix notification cleanup failures on post actions

[k3vin7]

Summary

• replace the MySQL-incompatible notification cleanup delete with a two-step keep-and-trim flow
• process notification events after commit in a new transaction so post likes/comments do not fail with 500
• add tests for the 100-notification boundary and listener failure isolation

Testing

• ./gradlew test --tests com.daramg.server.notification.application.NotificationServiceIntegrationTest --tests com.daramg.server.notification.event.NotificationEventListenerTest --tests com.daramg.server.notification.application.NotificationServiceUnitTest

[gemini-code-assist]

Code Review

This pull request refactors the AI comment scheduling and notification systems to use event-driven architecture with Spring's TransactionalEventListener. It introduces an AiReplyScheduleEvent and a new NotificationService method to handle notification creation and trimming asynchronously after transaction commits. Additionally, it implements a mechanism to limit the number of notifications per user to 100, ensuring that older notifications are pruned. I have no further feedback to provide on the implementation details.