feat: implement maintenance and not found error pages with appropriate messaging

Web/app.py

@@ -626,14 +626,18 @@ def handle_unexpected_exception(e):
     if request.is_json or request.path.startswith('/api/'):
         return jsonify({'error': 'Internal server error', 'status': 500}), 500
 
-    if 'username' in session:
+    try:
+        return render_template(
+            'maintenance.html',
+            error_code=500,
+            error_message='Das System führt gerade Wartungsarbeiten durch.',
+        ), 500
+    except Exception:
         try:
-            return render_template('error.html', error_code=500, error_message='Interner Serverfehler.'), 500
+            return render_template('error.html', error_code=500, error_message='Das System führt gerade Wartungsarbeiten durch.'), 500
         except Exception:
             return 'Internal Server Error', 500
 
-    return redirect(url_for('login'))
-
 
 def _csrf_error_response(message='CSRF token fehlt oder ist ungültig.'):
     if request.is_json or request.path.startswith('/api/') or request.path in {'/download_book_cover', '/proxy_image', '/log_mobile_issue'}:

Web/modules/terminplaner/blueprint.py

@@ -13,6 +13,14 @@ def _require_module_enabled():
         return redirect(url_for('home'))
     return None
 
+
+def _appointment_not_found_response():
+    return render_template(
+        'terminplaner_not_found.html',
+        error_code=404,
+        error_message='Der Termin wurde nicht gefunden.',
+    ), 404
+
 @appoint_bp.route('/client/<appointment_id>', methods=['POST', 'GET'])
 def client(appointment_id):
     """
@@ -24,8 +32,7 @@ def client(appointment_id):
 
     available = appointment_service.get_available(appointment_id)
     if not available:
-        flash('Der Termin wurde nicht gefunden.', 'error')
-        return redirect(url_for('terminplan'))
+        return _appointment_not_found_response()
 
     if request.method == 'POST':
         start_daytime = request.form.get('start_day_time')
@@ -113,8 +120,7 @@ def calendar_export(appointment_id):
 
     ics_content = appointment_service.build_calendar_ics(appointment_id)
     if not ics_content:
-        flash('Der Termin wurde nicht gefunden.', 'error')
-        return redirect(url_for('terminplaner.configure'))
+        return _appointment_not_found_response()
 
     response = Response(ics_content, mimetype='text/calendar; charset=utf-8')
     response.headers['Content-Disposition'] = f'attachment; filename=terminplan-{appointment_id}.ics'

Web/templates/maintenance.html

@@ -0,0 +1,28 @@
+{% extends "base.html" %}
+
+{% block title %}Wartungsarbeiten - Inventarsystem{% endblock %}
+
+{% block content %}
+<div class="container py-5">
+    <div class="row justify-content-center">
+        <div class="col-12 col-lg-8 col-xl-7">
+            <div class="card border-0 shadow-lg rounded-4 overflow-hidden">
+                <div class="card-header text-white" style="background: linear-gradient(135deg, #0f4c5c, #16697a);">
+                    <p class="text-uppercase small fw-semibold mb-1 opacity-75">Systemstatus</p>
+                    <h1 class="h3 mb-0 fw-bold">Wartungsarbeiten</h1>
+                </div>
+                <div class="card-body p-4 p-md-5 bg-white text-center">
+                    <div class="display-3 fw-bold text-warning mb-3">500</div>
+                    <p class="lead mb-2">Das System führt gerade Wartungsarbeiten durch.</p>
+                    <p class="text-muted mb-4">Bitte versuchen Sie es in wenigen Minuten erneut. Falls das Problem bestehen bleibt, informieren Sie bitte die Administration.</p>
+
+                    <div class="d-flex flex-column flex-sm-row justify-content-center gap-2">
+                        <a href="{{ url_for('login') }}" class="btn btn-primary btn-lg">Zum Login</a>
+                        <a href="javascript:history.back()" class="btn btn-outline-secondary btn-lg">Zurück</a>
+                    </div>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>
+{% endblock %}

Web/templates/terminplaner_not_found.html

@@ -0,0 +1,28 @@
+{% extends "base.html" %}
+
+{% block title %}Termin nicht gefunden - Inventarsystem{% endblock %}
+
+{% block content %}
+<div class="container py-5">
+    <div class="row justify-content-center">
+        <div class="col-12 col-lg-8 col-xl-7">
+            <div class="card border-0 shadow-lg rounded-4 overflow-hidden">
+                <div class="card-header text-white" style="background: linear-gradient(135deg, #0f4c5c, #16697a);">
+                    <p class="text-uppercase small fw-semibold mb-1 opacity-75">Terminplaner</p>
+                    <h1 class="h3 mb-0 fw-bold">Termin nicht gefunden</h1>
+                </div>
+                <div class="card-body p-4 p-md-5 bg-white text-center">
+                    <div class="display-3 fw-bold text-danger mb-3">404</div>
+                    <p class="lead mb-2">Der angeforderte Termin existiert nicht mehr oder der Link ist ungültig.</p>
+                    <p class="text-muted mb-4">Bitte prüfen Sie den Link erneut oder öffnen Sie den Terminplaner von der Startseite aus.</p>
+
+                    <div class="d-flex flex-column flex-sm-row justify-content-center gap-2">
+                        <a href="{{ url_for('home') }}" class="btn btn-primary btn-lg">Zur Startseite</a>
+                        <a href="javascript:history.back()" class="btn btn-outline-secondary btn-lg">Zurück</a>
+                    </div>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>
+{% endblock %}