Security, code quality and frontend improvements

- Move SECRET_KEY out of docker-compose into .env (env_file), add .env.example
- Add flask-limiter with 10 req/min on login route; introduce util/limiter.py
- Replace direct mysql.connector.connect() calls with MySQLConnectionPool via util/db_pool.py
- Fix deprecated datetime.utcnow() -> datetime.now(timezone.utc) in auth/login.py
- Remove dead /api/scripts 410 route from admin.py
- Add MD5 security warning in Md5Tool.jsx
- Add ErrorBoundary component and wrap App.jsx
- Expand README with setup guide, screenshot and project structure

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

frontend/src/components/Md5Tool.jsx

@@ -18,11 +18,16 @@ function Md5Tool() {
   return (
     <div className="main-content">
       <h2>MD5 Hasher</h2>
+      <p style={{ color: '#e07b00', fontSize: '0.875rem', marginBottom: '0.75rem' }}>
+        <strong>Sicherheitshinweis:</strong> MD5 ist kryptografisch unsicher und darf
+        nicht zum Hashen von Passwörtern verwendet werden. Für Passwörter bitte
+        bcrypt, Argon2 oder scrypt einsetzen.
+      </p>
       <input
         type="text"
         value={input}
         onChange={(e) => setInput(e.target.value)}
-        placeholder="Gib ein Passwort ein"
+        placeholder="Eingabe"
       />
       <button onClick={hashPassword}>Hash berechnen</button>
       {result && <p><strong>MD5:</strong> {result}</p>}