Entries list: filter by a field, so a console can filter a choice column

`GET /api/contents` filters by content type, status and search, and nothing else. barakoBrew#136 asks for the entries list to filter a choice column by option, and the console has no honest way to do it: filtering the page of 20 it holds would show the server's total beside rows it did not search, which is the reason #440 moved the other filters server side.

Saved queries match a choice by value (#820), but only on Public fields and only through a stored definition, so they do not cover an admin narrowing the entries list.

Asked for: `filter[field][op]=value` on `GET /api/contents` when `contentType` is set, with the delivery API's rules for a choice (`eq` and `ne` only on a list). The permission check and sensitivity scrub already run after the query, so a field filter can only remove rows, the same argument the status filter makes in `Features/Content/List/Endpoint.cs`.

barakoBrew filters a choice by option in saved queries for now and adds the entries list filter once this lands.

## Where it lives

Core: the authenticated contents list endpoint.

Start in Features/Content/List/Endpoint.cs, the authenticated contents list endpoint, and trace how the existing status filter is applied before permission checks and sensitivity scrubbing. Compare the delivery API's choice-filter rules, then verify that GET /api/contents accepts filter[field][op]=value with contentType set and supports eq and ne for choice fields without widening results.

Written by the indexing model from the issue text.