Fix medium/low priority review items; update README

Medium:
- Frontend: Error-Handling in fetchUsers/fetchApiKeys (try/catch)
- Frontend: Loading-Race behoben (Promise.all + .finally)
- Frontend: API-Keys maskiert (nur letzte 4 Zeichen sichtbar)
- Tests: Setup-Code aus test_auth.py in conftest.py konsolidiert
- Tests: Fixture-Scope vereinheitlicht (function statt session)

Low:
- bare except in database.py → except Exception
- datetime.utcnow → datetime.now(timezone.utc) durchgängig
- DateTime(timezone=True) in allen Modell-Spalten
- .gitignore hinzugefügt (.env, *.db, __pycache__, .idea, node_modules)

Docs:
- README aktualisiert (Sicherheit, Konfiguration, Projektstruktur, Tests)

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

frontend/src/main.jsx

@@ -2,33 +2,43 @@ import React, { useState, useEffect } from 'react';
 import axios from 'axios';
 import './styles.css';
 
+const maskKey = (key) => `••••••••${key.slice(-4)}`;
+
 function App() {
   const [users, setUsers] = useState([]);
   const [apiKeys, setApiKeys] = useState([]);
   const [loading, setLoading] = useState(true);
+  const [error, setError] = useState(null);
 
   useEffect(() => {
-    fetchUsers();
-    fetchApiKeys();
-    setLoading(false);
+    Promise.all([fetchUsers(), fetchApiKeys()]).finally(() => setLoading(false));
   }, []);
 
   const fetchUsers = async () => {
-    const res = await axios.get('/api/users');
-    setUsers(res.data);
+    try {
+      const res = await axios.get('/api/users');
+      setUsers(res.data);
+    } catch (err) {
+      setError('Benutzer konnten nicht geladen werden.');
+    }
   };
 
   const fetchApiKeys = async () => {
-    const res = await axios.get('/api/api-keys');
-    setApiKeys(res.data);
+    try {
+      const res = await axios.get('/api/api-keys');
+      setApiKeys(res.data);
+    } catch (err) {
+      setError('API-Keys konnten nicht geladen werden.');
+    }
   };
 
   if (loading) return <div>Loading...</div>;
+  if (error) return <div className="error">{error}</div>;
 
   return (
     <div className="container">
       <h1>Ollama Proxy Admin</h1>
-      
+
       <section>
         <h2>Users</h2>
         <table>
@@ -70,7 +80,7 @@ function App() {
               <tr key={key.id}>
                 <td>{key.id}</td>
                 <td>{key.name}</td>
-                <td>{key.key}</td>
+                <td>{maskKey(key.key)}</td>
                 <td>{key.user_id}</td>
                 <td>{key.is_active ? 'Active' : 'Inactive'}</td>
               </tr>
@@ -82,4 +92,4 @@ function App() {
   );
 }
 
-export default App;
+export default App;