refactor(caddy): modularise deployment init

Split the monolithic init-caddy.sh into four ordered, idempotent
setup scripts for the Caddy deployment environment:

- install-base-deps: install required apt packages
- install-aws-cli: bootstrap AWS CLI v2
- install-caddy: fetch binary, create caddy:caddy, register systemd unit
- prepare-caddy-cicd: create bot deploy user + SSH, hand over web root
  with ACLs, write site Caddyfile and start the service
- Make the web root overridable via WEB_ROOT (default /var/www)

Linux/install-base-deps.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+#
+# Base Dependencies Installation for Debian/Ubuntu
+# Installs the packages required by the other server initialisation scripts.
+#
+
+set -euo pipefail
+
+# Ensure script is executed with root privileges
+if [ "$(id -u)" -ne 0 ]; then
+    echo "Error: This script must be run as root." >&2
+    exit 1
+fi
+
+echo "==> Updating package index..."
+apt-get update -qq
+
+echo "==> Installing base dependencies (acl, libcap2-bin, rsync, curl, unzip)..."
+apt-get install -y acl libcap2-bin rsync curl unzip
+
+echo "========================================================="
+echo " Base dependencies installed successfully!"
+echo "========================================================="