Implement subscribe newsletter

[GideonBature]

This PR adds a complete newsletter subscription system to the application, allowing users to subscribe with their email and name through a REST API endpoint.

Changes Made

Backend Implementation

• New Module Structure: Created src/http/newsletter/ module with organized submodules:
  • domain.rs: Contains the NewsletterSubscriber struct
  • subscribe.rs: Handles subscription logic and database operations
  • mod.rs: Module organization and routing

API Endpoint

• POST /newsletter/subscribe: New endpoint for newsletter subscriptions
  • Accepts JSON payload with email and name fields
  • Validates email format and name length (2-255 characters)
  • Returns 200 OK on successful subscription
  • Returns 422 Unprocessable Entity for validation errors

Testing

All tests pass and provide comprehensive coverage of the happy path and error scenarios. Run tests with:

cargo test

Closes #104

[ONEONUORA]

Nice implementation @GideonBature Keep up the good work

[GideonBature]

Thank you ser!