Replace user-specific merge token with a dedicated service token

### Description

As a maintainer of the API Hub,
I want the daily merge process to use a dedicated service token instead of a user-specific token,
so that merges are not dependent on individual user credentials and improve security and maintainability.

### Acceptance Criteria

- [ ] The daily merge process uses a dedicated service token
- [ ] Documentation is updated to reflect the new token usage
- [ ] Legacy user-specific tokens are deprecated and removed from configuration

### Additional Information

Currently, the API Hub's daily merge process uses a token that appears to be tied to a specific user. This should be changed to a dedicated service account or machine token to avoid dependency on a single user's credentials and to follow best practices for automation and security.