ipx-directory: the directory of public podcasts ipx servers share (#204)

A new crate and service, crates/directory, live at ipx-dir.sdf1.net. ipx
servers send the public feeds their people subscribe to (POST /v1/snapshot,
replacing what that token sent before); the directory fetches each feed's
title, artwork, category, description and latest twenty items every six
hours with ipx-feed, and lists them A to Z and by subscribers summed over
every server. It holds metadata only, never a file or a file's address.

Tokens are in its config file as SHA-256 hashes, each ro, rw or rwa, and
the file is read again within a minute of a change, so adding and revoking
one is an edit and nothing else; a revoked token takes its server's counts.
At first they were to come from environment variables, which would have
meant recreating the container to add one. A file that does not load keeps
the tokens as they were.

Every address sent is checked with ipx_feed::url_looks_private, the check
ipx makes before sending, so one server's miss does not publish a paid feed
to every other; an rwa token can remove and block what both miss.

Deployed: ipx-directory in the ipx Arcane project, database ipx_directory
(its own login) on the databases project's Postgres, which db-backup
already covers, and a tunnel route with no Access in front. The store's
tests run on Postgres as well as SQLite when IPX_TEST_DATABASE_URL is set.

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

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = ["crates/ipx", "crates/feed"]
+members = ["crates/ipx", "crates/feed", "crates/directory"]
 resolver = "3"
 
 # One version for the whole workspace: what a release sets, and what the logo's tooltip shows.
@@ -9,6 +9,7 @@ edition = "2024"
 
 [workspace.dependencies]
 ipx-feed = { path = "crates/feed" }
+ipx-directory = { path = "crates/directory" }
 ammonia = "4.1.4"
 anyhow = "1.0.104"
 argon2 = "0.6.0"