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>

.env.example

@@ -2,3 +2,6 @@ MYSQL_ROOT_PASSWORD=sproutly_root_secret
 MYSQL_USER=sproutly
 MYSQL_PASSWORD=sproutly_secret
 SECRET_KEY=your-secret-key-change-this
+
+ADMIN_EMAIL=admin@example.com
+ADMIN_PASSWORD=change-this-password