Add super admin panel and update README

- Admin account bootstrapped from ADMIN_EMAIL/ADMIN_PASSWORD env vars on startup
- Admin panel: list users, view content, reset passwords, disable/delete accounts
- is_admin and is_disabled columns on users table
- Disabled accounts blocked at login
- README updated with admin setup instructions and panel docs

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

docker-compose.yml

@@ -33,6 +33,8 @@ services:
       DB_USER: ${MYSQL_USER}
       DB_PASSWORD: ${MYSQL_PASSWORD}
       SECRET_KEY: ${SECRET_KEY}
+      ADMIN_EMAIL: ${ADMIN_EMAIL}
+      ADMIN_PASSWORD: ${ADMIN_PASSWORD}
     depends_on:
       mysql:
         condition: service_healthy