Choose the Stalwart by the domain somebody signs in with

One ihasmail in front of several Stalwarts, from #238. STALWART_URL stays
required and stays the default, so an installation that sets nothing behaves
exactly as it always has -- the mapping only adds domains that go elsewhere.

An unlisted domain goes to the default. So does a bare username, which
Stalwart accepts and which has no domain to map at all.

A listed domain never falls back. If its server is unreachable that sign-in
fails rather than retrying against the default, because falling back would
authenticate somebody against a server their domain was deliberately routed
away from -- and if the same account name existed there, they would land in
another tenant's mailbox. The fallback is a decision about unmapped domains,
taken before any network call, not a recovery path.

Smaller than it sounds because only four places read config.stalwartUrl, all
in upstream.ts. The upstream session now records which server issued it, since
the relative URLs inside it only mean anything against that server, and every
route already holding a session gets the right upstream without a second
lookup. The client is untouched: it talks to one proxy and never learns there
is more than one server behind it, which is exactly why this is small and
several-servers-at-once is not.

The upstream is derived from the username rather than stored on the session,
so a mapping change takes effect on restart instead of being frozen into
sessions that outlive it.

Validated at boot the way the settings policy is: malformed JSON, a duplicate
domain once normalised, a missing file or a value that is not an http(s) URL
all stop the server. Domains are lower-cased and stripped of a trailing dot,
because that is how one arrives off a username and comparing them any other
way means a mapping that silently never matches. The servers themselves are
not contacted -- a mapping is a routing table, not a health check, and one
customer's outage must not stop ihasmail starting for the other four.

Eight tests on the routing, two on the shipped example, and the four refusals
checked by hand against a real config load.

.env.example

@@ -93,3 +93,17 @@ SOURCE_URL=https://github.com/Coffey-Labs/ihasmail
 #
 # Read once at startup: editing a policy means restarting the container.
 # Docs: https://docs.ihasmail.org/configure/#settings-your-installation-decides
+
+# ---- Several Stalwart servers (optional) ----
+#
+# Choose the upstream by the domain someone signs in with. STALWART_URL above
+# stays required and stays the default; this only adds domains that go
+# elsewhere. See the shipped stalwart-servers.example.json, and mount it
+# read-only:
+#
+#   -v /srv/ihasmail/servers.json:/etc/ihasmail/servers.json:ro
+#
+# STALWART_SERVERS_FILE=/etc/ihasmail/servers.json
+#
+# An unlisted domain, or a username with no domain, goes to STALWART_URL. A
+# listed domain never falls back. Read once at startup: editing means a restart.