gleam_stdlib now requires >= 1.14.0, breaking the v1.7.0 pin during
build. Bump to v1.16.0 — a stable release with several weeks of bake
time rather than today's v1.17.0.
A small Gleam HTTP server in signup/ exposes POST /signup that validates
a shared INVITE_CODE env var, then PUTs a user document to CouchDB's
_users database using the existing admin credentials. The frontend gets
back 201 + the username and is expected to call /_session itself to
obtain the AuthSession cookie (chosen over server-side cookie forwarding
because vaquant.at and couch.apoena.dev are different sites and the
cross-site cookie path is fragile under modern browser privacy modes).
Wired into docker-compose as a second service, reachable on
signup-couch.apoena.dev with its own Traefik labels and the same
hardcoded UUID network attachment as couchdb. Reuses COUCHDB_USER /
COUCHDB_PASSWORD for the admin call. INVITE_CODE must be set in Coolify
env vars.
