[#13320] Refactoring data deletion logic to enhance maintainability and debugging

[rainboh0622]

Fixes #13320

**Outline of Solution

1. Successfully completed deletion method refactoring, consolidating deletion methods from multiple Logic classes into DeletionService.
2. Reworked soft-delete methods into Db classes that did not already contain them.
3. Successfully completed all modifications related to deletion preview as required by the proposal.

Modified Classes:

1. DeletionService.java (created)

• Unified entry point for all deletion operations

2. DeletionPreviewData.java

• Core data model, including:
  • Entity type and identifier
  • Count of affected entities (courses, students, instructors, feedback sessions, etc.)
  • Warning messages list
  • Cascaded deletion information
  • Critical flags (whether last instructor, whether will orphan course, etc.)

3. DeletionPreviewService.java

• Deletion preview service, providing the following preview methods:
  • previewCourseDeletion() - Preview course deletion
  • previewStudentDeletion() - Preview student deletion
  • previewInstructorDeletion() - Preview instructor deletion
  • previewAccountDeletion() - Preview account deletion
  • previewAccountRequestDeletion() - Preview account request deletion
  • previewNotificationDeletion() - Preview notification deletion

4. LogicStarter.java

• Initialises DeletionService.java and DeletionPreviewService.java.

5. _Logic classes

• Refactored Deletion methods into DeletionService.java, including:
  • AccountRequestsLogic.java
  • AccountsLogic.java
  • CoursesLogic.java
  • DataBundleLogic.java
  • DeadlineExtensionsLogic.java
  • FeedbackQuestionsLogic.java
  • FeedbackResponseCommentsLogic.java
  • FeedbackResponsesLogic.java
  • InstructorsLogic.java
  • NotificationsLogic.java
  • StudentsLogic.java

6. _Db classes

• Added softDelete_() and restoreDeleted_() methods, including:
  • AccountRequestsDb.java
  • AccountsDb.java
  • DeadlineExtensionsDb.java
  • FeedbackQuestionsDb.java
  • FeedbackResponseCommentsDb.java
  • FeedbackResponsesDb.java
  • InstructorsDb.java
  • NotificationsDb.java
  • StudentsDb.java

7. Entity classes & attribute classes

• Added deletedAt Instant and relevant methods into:

  • Entity classes in main/java/teammates/storage/entity/
  • BaseEntity.java in main/java/teammates/storage/sqlentity/
  • Atttribute classes in main/java/teammates/common/datatransfer/attributes/

• Linked deletedAt Instants between corresponding entity/atttribute classes by editing valueOf().

8. Database Layer Enhancements

• Added count methods to the following Db classes to support preview functionality:
  • FeedbackSessionsDb: Added getFeedbackSessionsCountForCourse()
  • FeedbackQuestionsDb: Added getFeedbackQuestionsCountForCourse()
  • FeedbackResponsesDb: Added getFeedbackResponsesCountForCourse()
  • FeedbackResponseCommentsDb: Added getFeedbackResponseCommentsCountForCourse()
  • DeadlineExtensionsDb: Added getDeadlineExtensionsCountForCourse()

9. API Endpoints Layer

• Created the following REST API endpoints:
  • PreviewCourseDeletionAction - GET /webapi/course/deletion/preview
  • PreviewStudentDeletionAction - GET /webapi/student/deletion/preview
  • PreviewInstructorDeletionAction - GET /webapi/instructor/deletion/preview
  • PreviewAccountDeletionAction - GET /webapi/account/deletion/preview

10. Output Format

• DeletionPreviewOutput.java. API response format, containing:
  • Statistics of all affected entities
  • Warning messages
  • Cascaded deletion details
  • Well-structured data for frontend display

11. Routing Configuration

• Added new URI constants in Const.java
• Registered all preview API endpoints in ActionFactory.java
• Initialized DeletionPreviewService.java dependencies in LogicStarter.java

12. Created or modified basic test framework:

• DeletionServiceTest.java - Deletion service unit tests
• DeletionPreviewServiceTest.java - Service layer unit tests
• PreviewCourseDeletionActionTest.java - API endpoint test template
• _DbTest.java in test/java/teammates/storage/api/ - Soft delete unit tests

[github-actions]

Hi @rainboh0622, thank you for your interest in contributing to TEAMMATES!
However, your PR does not appear to follow our contribution guidelines:

• Title must start with the issue number the PR is fixing in square brackets, e.g. [#<issue-number>]

Please address the above before we proceed to review your PR.

[u7676784]

Update PR title to start with [#13320] (add a space between it and the rest of title?)