AGPL source offer and name cleanup

Every build writes the exact source it was built from, uncommitted work and
new files included, as source.tar.gz next to the app, named after that tree.
Docker builds, which have no git, pack the build context and name it by a
hash of its files. The sign-in page and Settings > About link to it instead of
a repository that can drift.

What users, operators and packagers see no longer names the upstream server:
- interface text, in all nine catalogues, with a token-session line for
  Security;
- server messages;
- the settings, now MAIL_SERVER_URL, MAIL_SERVERS_FILE, ADMIN_URL and
  MAIL_SERVER_FOLLOW_ADVERTISED_URLS, and mail-servers.example.json;
- the Tenants notice, which is gone;
- the README, CONTRIBUTING and SECURITY.

ihasmail's own FEATURES, KNOWN-ISSUES and ROADMAP stay with public ihasmail,
and INBUXA.md is folded into the README.

server/src/domainrouting.test.ts

@@ -1,7 +1,7 @@
 import { test } from "node:test";
 import assert from "node:assert/strict";
 
-process.env.STALWART_URL = "https://default.example";
+process.env.MAIL_SERVER_URL = "https://default.example";
 
 const { upstreamFor } = await import("./upstream.js");
 const { config } = await import("./config.js");
@@ -9,7 +9,7 @@ const { config } = await import("./config.js");
 /**
  * Which Stalwart a username goes to (#238).
  *
- * `STALWART_URL` is required and is the default. The mapping only adds domains
+ * `MAIL_SERVER_URL` is required and is the default. The mapping only adds domains
  * that go elsewhere, so an installation with no mapping behaves exactly as it
  * always has -- which is what these first cases pin.
  */