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
directory's 403 reaches the page as 403, not 502.

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

crates/directory/CHANGELOG.md

@@ -12,5 +12,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `GET /v1/instances` lists the servers holding a snapshot, with their feeds and subscribers, for an admin token (#206).
 - `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, each feed listed once it has been read. Tokens are in its config file, each read-only, read-write or admin, and revoked one at a time (#204, #205).
 - Traces: a span a request, joining the trace of the ipx server that called, and a trace a refresher round with a span a feed (#213).