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>

mysql/init.sql

@@ -4,6 +4,8 @@ CREATE TABLE IF NOT EXISTS users (
     id INT AUTO_INCREMENT PRIMARY KEY,
     email VARCHAR(255) NOT NULL UNIQUE,
     hashed_password VARCHAR(255) NOT NULL,
+    is_admin BOOLEAN NOT NULL DEFAULT FALSE,
+    is_disabled BOOLEAN NOT NULL DEFAULT FALSE,
     created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
 );