Add applicant subject-area subscription server

[CelineLahnor]

Description

Applicants should be able to persist one or more subject-area subscriptions for future notification logic. This issue covers the persistence model, server API, authorization, and validation only. It does not include UI or sending emails.

Tasks

• Add a persistence model for applicant subject-area subscriptions
• Add a Liquibase migration for the new table
• Add a unique constraint to prevent duplicate subscriptions per applicant and subject area
• Implement repository, service, and DTOs
• Add GET and PUT endpoints for the authenticated applicant
• Ensure only applicants can read and update their own subscriptions
• Reuse the existing subject-area option source for validation
• Add server tests for loading, saving, deduplication, validation, and authorization

Acceptance Criteria

• Applicants can store multiple subject-area subscriptions
• The authenticated applicant can load their saved subscriptions via API
• The authenticated applicant can update their saved subscriptions via API
• Duplicate subject areas are not stored twice
• Non-applicant roles cannot use this applicant-specific setting
• Existing notification settings are unchanged
• Relevant backend tests are implemented and passing