Add proper login to Admin UI, replace Caddy basic_auth

- iron-session for encrypted httpOnly session cookies
- POST /api/auth/login: verifies ADMIN_USERNAME/ADMIN_PASSWORD, sets session, returns API key
- POST /api/auth/logout: destroys session
- middleware.ts: protects all routes, redirects to /login if unauthenticated
- Login page: username + password form (no more browser popup)
- Sidebar: logout calls API route and clears session
- docker-compose.prod.yml: admin-ui reads /opt/rustycms/.env.admin
- deploy.yml: generates .env.admin from Gitea secrets
- Caddy: basic_auth removed from /admin* block

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -19,6 +19,7 @@ services:
     restart: unless-stopped
     expose:
       - "3001"
+    env_file: /opt/rustycms/.env.admin
     networks:
       - web