Knowledge article categories now use a missing database index, helping searches, filters, grouping, and related cleanup actions run more efficiently. This should improve performance in common Knowledge views without changing how users work.

Add missing index on `category` of `knowledge.article`, used in many filters of the search view and the group by `category`. Also used in a few methods like `_check_sidebar_display` and during unlinking of partners.

task-3420528

Several app pages now correctly report access and missing-page errors instead of treating them as successful requests. This helps users see the right error message and prevents confusing behavior in affected workflows.

Some json-controllers were returning http exceptions (e.g. NotFound, Forbidden) instead of raising them causing the RPC error handler in the JS framework to ignore them.