feat: Ability to clean up orphaned model permissions

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Problem Description

I reported this as a bug, but apparently it's just a lack of feature, even though I think the inability to maintain referential integrity in a database is pretty much a bug IMHO. Either way, the issue is the same.

https://github.com/open-webui/open-webui/issues/26788#issuecomment-4887719536

As soon as a model stops being available all of its permissions become inaccessible and manageable. It provides confusion (listing of hypothetical model counts and permission counts for users) and a potential security risk when a model name gets reused and it immediately grants users of the prior use of it access.

Proposed Solution

List every model, active or offline in the model admin panel so action can be taken on them, permissions adjusted for them or flat out purge them from the system.

Alternatives Considered

Purge permissions if a model becomes unavailable, having a source-model level uniqueness.

Realistically there is no better alternative than to give admins the tools to properly manage access.

Additional Context

No response