Add admin instance management and system tools

- Implemented sync_dev_hosts.sh for managing local hosts entries for development.
- Created admin_instances.html for managing school instances, including creation and listing.
- Developed admin_system.html for core services management, including restart and backup operations.
- Added my_instance.html for users to view and manage their assigned instances.
- Introduced provision_instance.sh for provisioning new instances with Docker, including Nginx configuration and SSL certificate handling.

Website/launch_dev.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "$SCRIPT_DIR"
+
+export SESSION_COOKIE_SECURE="0"
+export INSTANCE_TLS_MODE="development"
+export INSTANCE_PARENT_DOMAIN="${INSTANCE_PARENT_DOMAIN:-meine-domain}"
+
+echo "Launching DEVELOPMENT stack"
+echo "  SESSION_COOKIE_SECURE=$SESSION_COOKIE_SECURE"
+echo "  INSTANCE_TLS_MODE=$INSTANCE_TLS_MODE"
+echo "  INSTANCE_PARENT_DOMAIN=$INSTANCE_PARENT_DOMAIN"
+
+docker compose build website
+docker compose up -d
+
+if [[ -x "$SCRIPT_DIR/sync_dev_hosts.sh" ]]; then
+	"$SCRIPT_DIR/sync_dev_hosts.sh" || true
+fi
+
+echo "Development stack is running on http://localhost:4999"
+echo "Provisioning creates self-signed certs per subdomain when needed."