Migrate to mcdsl: auth, config, csrf, web

- Replace internal/auth with mcdsl/auth
- Replace internal/config with mcdsl/config (embed config.Base)
- Replace internal/webserver/csrf.go with mcdsl/csrf
- Use mcdsl/web for session cookies and template rendering
- Use mcdsl/httpserver for server setup and StatusWriter
- Remove direct mcias client library dependency
- Update .golangci.yaml to v2 format (formatters section)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/scripts/install.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+set -eu
+
+SERVICE=mcat
+SRV_DIR="/srv/${SERVICE}"
+BIN_DIR="/usr/local/bin"
+
+# Create system user (idempotent).
+if ! id "${SERVICE}" >/dev/null 2>&1; then
+    useradd --system --shell /usr/sbin/nologin --home-dir "${SRV_DIR}" "${SERVICE}"
+    echo "Created system user: ${SERVICE}"
+fi
+
+# Install binary.
+install -m 0755 "${SERVICE}" "${BIN_DIR}/${SERVICE}"
+echo "Installed ${BIN_DIR}/${SERVICE}"
+
+# Create data directory structure.
+install -d -o "${SERVICE}" -g "${SERVICE}" -m 0700 "${SRV_DIR}"
+install -d -o "${SERVICE}" -g "${SERVICE}" -m 0700 "${SRV_DIR}/certs"
+
+# Copy example config if none exists.
+if [ ! -f "${SRV_DIR}/${SERVICE}.toml" ]; then
+    install -o "${SERVICE}" -g "${SERVICE}" -m 0600 \
+        deploy/examples/${SERVICE}.toml.example "${SRV_DIR}/${SERVICE}.toml"
+    echo "Installed example config to ${SRV_DIR}/${SERVICE}.toml"
+fi
+
+# Install systemd units.
+install -m 0644 deploy/systemd/${SERVICE}.service /etc/systemd/system/
+systemctl daemon-reload
+echo "Installed systemd unit. Enable with: systemctl enable --now ${SERVICE}"