CVE-2026-44558: Open WebUI's Channel Access Grants Bypass filter_allowed_access_grants

All resource routers in Open WebUI (knowledge, models, notes, prompts, tools, skills) call filter_allowed_access_grants() before persisting access grants. This function strips principal_id: "*" wildcard grants from users who lack the relevant sharing.public_* permission, and strips individual user grants from users who lack access_grants.allow_users permission.

The channel router does not call filter_allowed_access_grants on either create or update paths. A non-admin user who can create group channels (or who owns a channel) can submit arbitrary access grants — including public wildcard grants — and those grants are stored verbatim, bypassing the admin’s permission framework.