Add registration toggle and password change; genericise deploy

Two things the running instance needed.

**Registration toggle.** Admin -> General, backed by a new settings table
group rather than the environment. LEMBAS_ALLOW_SIGNUP now seeds only the
initial value: once an administrator saves the setting, the stored value
wins. The alternative -- environment always winning -- means a toggle in
the UI silently reverts on the next restart, which is worse than not
offering one. Closing registration also removes the "Create one" link
from the sign-in page, so the link never leads somewhere that refuses.

**Password change**, on the user settings page. Changing a password
revokes every other session and immediately re-issues a cookie for the
current one: if the reason for the change is that somebody else knows
the password, leaving their session alive defeats the point, but signing
the user out of the tab they are standing in is merely rude.

**deploy/ is now host-agnostic.** This repository is public, so the unit
and vhost became templates with __PREFIX__ / __SITE_HOST__ / __APP_PORT__
substituted at install time, and every path, hostname and port moved to
environment variables. REPO_URL defaults to the checkout's own origin so
a fork deploys itself. Machine-specific values belong in private notes,
not here -- CLAUDE.md now says so.

83 tests, ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/update.sh

@@ -1,36 +1,39 @@
 #!/usr/bin/env bash
-# Pull the latest LLeMbas and restart the service.
+# Pull the latest LLeMbas into the deployment and restart the service.
 #
-# This is what to run after pushing: it fetches, hard-resets the deployment
-# checkout to the remote branch, reinstalls dependencies if they changed, and
-# restarts. Nothing is ever edited in place at /srv/lembas/app, so a hard reset
-# is safe and avoids merge conflicts from a dirty deployment tree.
+# Run this after pushing. It fetches, hard-resets the deployment checkout to the
+# remote branch, reinstalls dependencies if they changed, and restarts. Nothing
+# is ever edited in place under the deployment prefix, so a hard reset is safe
+# and avoids merge conflicts from a dirty tree.
 set -euo pipefail
 
-SERVICE_USER=lembas
-PREFIX=/srv/lembas
+SERVICE_USER="${SERVICE_USER:-lembas}"
+PREFIX="${PREFIX:-/srv/lembas}"
+BRANCH="${LEMBAS_BRANCH:-main}"
+
 APP="$PREFIX/app"
 VENV="$PREFIX/venv"
-BRANCH="${LEMBAS_BRANCH:-main}"
 
 if [[ ! -d "$APP/.git" ]]; then
   echo "No deployment at $APP. Run deploy/install.sh first." >&2
   exit 1
 fi
 
-before=$(sudo -u "$SERVICE_USER" git -C "$APP" rev-parse HEAD)
+git_as() { sudo -u "$SERVICE_USER" git -C "$APP" "$@"; }
+
+before=$(git_as rev-parse HEAD)
 
 echo "== fetching =="
-sudo -u "$SERVICE_USER" git -C "$APP" fetch --quiet origin "$BRANCH"
-sudo -u "$SERVICE_USER" git -C "$APP" reset --hard --quiet "origin/$BRANCH"
+git_as fetch --quiet origin "$BRANCH"
+git_as reset --hard --quiet "origin/$BRANCH"
 
-after=$(sudo -u "$SERVICE_USER" git -C "$APP" rev-parse HEAD)
+after=$(git_as rev-parse HEAD)
 
 if [[ "$before" == "$after" ]]; then
-  echo "   already at $(git -C "$APP" rev-parse --short HEAD), nothing to pull"
+  echo "   already at ${after:0:7}, nothing to pull"
 else
-  echo "   $(echo "$before" | cut -c1-7) -> $(echo "$after" | cut -c1-7)"
-  sudo -u "$SERVICE_USER" git -C "$APP" --no-pager log --oneline "$before..$after" | sed 's/^/     /'
+  echo "   ${before:0:7} -> ${after:0:7}"
+  git_as --no-pager log --oneline "$before..$after" | sed 's/^/     /'
 fi
 
 # Cheap and idempotent; catches a dependency added since the last deploy.
@@ -42,7 +45,7 @@ sudo systemctl restart lembas
 sleep 2
 
 if systemctl is-active --quiet lembas; then
-  echo "   lembas is running at https://chat.lan"
+  echo "   lembas is running"
 else
   echo "   lembas FAILED to start:" >&2
   sudo journalctl -u lembas -n 30 --no-pager >&2