Administer the directory from iPX with an rwa token (#206)

A Directory feed's page offers Check now and Remove to an admin whose token is rwa (the page
learns it from can_admin on the feed's page, asked of the directory only for admins). Admin,
Directory lists the servers sharing, from a new GET /v1/instances, and the blocked feeds by
title: a feed is often blocked because its address carries a key, so the address stays in the
directory. Unblocking or dropping a snapshot sends this server's snapshot again at once; since
#211 an unchanged one would otherwise wait half an hour, and an unblocked feed with it. The
directory's 403 reaches the page as 403, not 502.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- With a directory token that may administer it, an admin removes a feed from the directory (and blocks it), checks one now, sees and unblocks what is blocked, and sees and drops the servers sharing, from iPX (#206).
 - The Grafana dashboard is laid out by container: an overview of all three, Tower's load, CPU, memory and disks, then ipx-web (from its new request log), ipx's API, worker and process, and the directory, each with its own warnings (#210).
 - `ipx-directory`, a directory of public podcasts that ipx servers share: each sends the public feeds its people subscribe to, and any server with a token browses them. Tokens are in its config file, each read-only, read-write or admin, and revoked one at a time (#204).