Open WebUI: Non-admin users can delete admin-owned external knowledge connections via knowledge base deletion

## Summary External knowledge connections are created and owned by administrators, and are shared by every external knowledge base bound to them. Deleting an external knowledge base also removed that connection from the instance configuration, with no check on the caller's role and no check for other knowledge bases still using it. Any authenticated user holding a write grant on a single external knowledge base could therefore wipe a shared connection that other knowledge bases depend on, and the dedicated administrator route for deleting a connection explicitly refuses that same operation while the connection is still in use. ## Preconditions The deployment uses external knowledge bases, backed by the qdrant, milvus or pgvector external providers. Instances with only local knowledge bases are not affected. An administrator created at least one external connection and at least one external knowledge base bound to it. Both of those actions are admin-only. The attacker is an ordinary authenticated user holding a write grant on one of those external knowledge bases. A read grant is not enough, and an unrelated user cannot reach the route at all. The damage scales with sharing. It is worst when one connection backs several knowledge bases, because deleting a single knowledge base destroys the connection for all of them. ## Impact An ordinary user removes instance-wide configuration that only administrators can create or manage. Every other external knowledge base bound to the del