Feat: Added custom repository functions in NewsletterRepository and endpoints

[tachtwitch]

PR Description

This PR resolves the incorrect configuration of the NewsletterRepository interface and implements CRUD features, including retrieving newsletters by user ID, retrieving newsletters created after a specified date, and deleting newsletters by user ID.

Bug fixes:

*Resolved incorrect configuration of NewsletterRepository interface

##New features:

• Implemented CRUD features for NewsletterRepository interface
• Added content and title fields to @Entity newsletter
• Added unit tests for NewsletterRepository methods

Expected Behavior

The NewsletterRepository should correctly perform CRUD operations using the defined @Query annotations.

getNewslettersByUserId Endpoint

getNewslettersAfterDate Endpoint

Testing

Test 1: Retrieve newsletters by user ID

• Unit test: passed

Test 2: Retrieve newsletters created after a specified date

• Unit test: passed

Test 3: Delete newsletter by user ID

• Unit test: passed