Webhook BasicAuth Credentials Exposed to Read-Only Project Collaborators via API

Vikunja is an open-source self-hosted task management platform. Prior to version 2.2.1, the `GET /api/v1/projects/:project/webhooks` endpoint returns webhook BasicAuth credentials (`basic_auth_user` and `basic_auth_password`) in plaintext to any user with read access to the project. While the existing code correctly masks the HMAC `secret` field, the BasicAuth fields added in a later migration were not given the same treatment. This allows read-only collaborators to steal credentials intended for authenticating against external webhook receivers. Version 2.2.1 patches the issue.