Initial madcat-caddy: Caddy + Cloudflare DNS + caddy-security

Forked from tengu-apps/tengu-caddy, adding:
- caddy-security plugin (OAuth 2.0, Basic Auth, JWT)
- Gitea apt registry publishing in CI
- Linux-only builds (arm64 + amd64)
- Replaces/conflicts with caddy and tengu-caddy packages

debian/postinst

@@ -0,0 +1,24 @@
+#!/bin/sh
+set -e
+
+# Create caddy user/group if they don't exist
+if ! getent group caddy >/dev/null 2>&1; then
+    groupadd --system caddy
+fi
+
+if ! getent passwd caddy >/dev/null 2>&1; then
+    useradd --system --gid caddy --create-home --home-dir /var/lib/caddy --shell /usr/sbin/nologin caddy
+fi
+
+# Create config and data directories
+mkdir -p /etc/caddy
+mkdir -p /var/lib/caddy/.config/caddy
+mkdir -p /var/lib/caddy/.local/share/caddy
+chown -R caddy:caddy /var/lib/caddy
+
+# Enable and start the service
+if [ -d /run/systemd/system ]; then
+    systemctl daemon-reload
+    systemctl enable caddy.service
+    systemctl start caddy.service || true
+fi