refactor(auth): extract token handling to JWTManager

[kimitrii]

Changes Made

This PR extracts the JWT token generation logic from UserAuthService into a dedicated JWTManager class, improving reusability and simplifying token generation across multiple endpoints by centralizing the logic.

Changes Type

• Refactor

Checklist:

• The changes do not generate new error logs or warnings.
• I have added tests that prove the fix or new feature works as expected.
• Both new and existing tests pass locally.