Deprecate and remove the Dataset Manager role

[hotzenklotz]

Detailed Description

The Dataset Manager role overlaps with existing roles and is a source of confusion for users and administrators. Removing it is part of a broader effort to simplify the role system down to three core roles: Admin, Team Manager (or Team Admin), and Member.

Proposed change

Remove the Dataset Manager role. Existing users with this role should be migrated in a way that preserves their current dataset access.

Migration

One approach under consideration: move affected users into a new, dedicated team located in the root folder. This would preserve their existing access rights within the simplified role model. Migration behavior must be validated carefully to avoid unintended access changes.

Open questions

• Should the remaining manager role be called Team Manager or Team Admin? Something else
• Are there edge cases where the root-folder team migration could break access for existing Dataset Managers?
• Does any external integration or API consumer depend on the Dataset Manager role string?

Context

• Specific to long-running jobs (set jobsEnabled=true in application.conf)
• Specific to webknossos.org (set isDemoInstance=true in application.conf)