Enhance UserService with security features

[coderabbitai]

As discussed in PR #8, the UserService interface lacks essential security operations:

1. Password management:

   • ChangePassword(ctx context.Context, oldPassword, newPassword string) error
   • RequestPasswordReset(ctx context.Context, email string) error
   • ResetPassword(ctx context.Context, token, newPassword string) error

2. Session management:

   • CreateSession(ctx context.Context, user *model.User) (*model.Session, error)
   • InvalidateSession(ctx context.Context, sessionID string) error

3. Rate limiting for sensitive operations

Implementing these methods will enhance the security of the application.

Requested by @itsLeonB.

See the discussion at: #8 (comment)