feat: add Etherpad

Adds a new LXC helper-script for Etherpad (https://etherpad.org), a
real-time collaborative document editor (Node.js / pnpm).

- ct/etherpad.sh — launcher + update_script using check_for_gh_release
  against ether/etherpad-lite.
- install/etherpad-install.sh — Node.js 22 via NodeSource, pnpm via
  corepack, clone latest release tag, pnpm install --frozen-lockfile &&
  pnpm run build:etherpad, dedicated etherpad system user, systemd unit
  running `pnpm run prod`.
- json/etherpad.json — category 12 (Documents & Notes), port 9001,
  default credentials none, default resources 2 vCPU / 2 GB RAM / 8 GB
  disk on Debian 12 unprivileged.

Refs ether/etherpad#7529

ct/etherpad.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+source <(curl -fsSL https://raw.githubusercontent.com/community-scripts/ProxmoxVED/main/misc/build.func)
+# Copyright (c) 2021-2026 community-scripts ORG
+# Author: John McLear (JohnMcLear)
+# License: MIT | https://github.com/community-scripts/ProxmoxVED/raw/main/LICENSE
+# Source: https://etherpad.org
+
+APP="Etherpad"
+var_tags="${var_tags:-docs;collaboration;editor}"
+var_cpu="${var_cpu:-2}"
+var_ram="${var_ram:-2048}"
+var_disk="${var_disk:-8}"
+var_os="${var_os:-debian}"
+var_version="${var_version:-12}"
+var_unprivileged="${var_unprivileged:-1}"
+
+header_info "$APP"
+variables
+color
+catch_errors
+
+function update_script() {
+  header_info
+  check_container_storage
+  check_container_resources
+
+  if [[ ! -d /opt/etherpad-lite ]]; then
+    msg_error "No ${APP} Installation Found!"
+    exit
+  fi
+
+  if check_for_gh_release "etherpad-lite" "ether/etherpad-lite"; then
+    msg_info "Stopping Service"
+    systemctl stop etherpad
+    msg_ok "Stopped Service"
+
+    msg_info "Backing up Configuration"
+    [ -f /opt/etherpad-lite/settings.json ] && cp /opt/etherpad-lite/settings.json /opt/etherpad-settings.json.bak
+    [ -d /opt/etherpad-lite/var ] && cp -a /opt/etherpad-lite/var /opt/etherpad-var.bak
+    msg_ok "Backed up Configuration"
+
+    LATEST_TAG=$(curl -fsSL https://api.github.com/repos/ether/etherpad-lite/releases/latest | grep -oP '"tag_name":\s*"\K[^"]+')
+    msg_info "Updating to ${LATEST_TAG}"
+    cd /opt/etherpad-lite
+    $STD git fetch --tags --prune
+    $STD git checkout "${LATEST_TAG}"
+    export COREPACK_ENABLE_DOWNLOAD_PROMPT=0
+    $STD corepack enable
+    $STD pnpm install --frozen-lockfile
+    $STD pnpm run build:etherpad
+    msg_ok "Updated to ${LATEST_TAG}"
+
+    msg_info "Restoring Configuration"
+    [ -f /opt/etherpad-settings.json.bak ] && mv /opt/etherpad-settings.json.bak /opt/etherpad-lite/settings.json
+    [ -d /opt/etherpad-var.bak ] && rm -rf /opt/etherpad-lite/var && mv /opt/etherpad-var.bak /opt/etherpad-lite/var
+    chown -R etherpad:etherpad /opt/etherpad-lite
+    msg_ok "Restored Configuration"
+
+    msg_info "Starting Service"
+    systemctl start etherpad
+    msg_ok "Started Service"
+    msg_ok "Updated successfully!"
+  fi
+  exit
+}
+
+start
+build_container
+description
+
+msg_ok "Completed Successfully!\n"
+echo -e "${CREATING}${GN}${APP} setup has been successfully initialized!${CL}"
+echo -e "${INFO}${YW} Access it using the following URL:${CL}"
+echo -e "${TAB}${GATEWAY}${BGN}http://${IP}:9001${CL}"