Fix payment system balance integration and add calendar functionality

- Implement automated payment tracking with Django signals
- Fix duplicate transaction creation with unique referenz system
- Add calendar system with CRUD operations and event management
- Reorganize navigation menu (rename sections, move admin functions)
- Replace Geschichte editor with EasyMDE markdown editor
- Add management commands for balance reconciliation
- Create missing transactions for previously paid payments
- Ensure account balances accurately reflect all payment activity

Features added:
- Calendar entries creation and administration via menu
- Payment status tracking with automatic balance updates
- Duplicate prevention for payment transactions
- Markdown editor with live preview for Geschichte pages
- Database reconciliation tools for payment/balance sync

Bug fixes:
- Resolved IntegrityError on payment status changes
- Fixed missing account balance updates for paid payments
- Prevented duplicate balance deductions on re-saves
- Corrected menu structure and admin function placement

app/templates/stiftung/kalender/detail_old.html

@@ -0,0 +1,32 @@
+{% extends "base.html" %}
+
+{% block title %}{{ title }}{% endblock %}
+
+{% block content %}
+<div class="container-fluid">
+    <div class="row">
+        <div class="col-12">
+            <h1><i class="fas fa-calendar-alt me-2"></i>{{ title }}</h1>
+        </div>
+    </div>
+    
+    <div class="row">
+        <div class="col-lg-8">
+            <div class="card shadow">
+                <div class="card-body">
+                    <div class="alert alert-info" role="alert">
+                        <i class="fas fa-info-circle me-2"></i>
+                        Termin-Details werden hier angezeigt, sobald die Funktionalität vollständig implementiert ist.
+                    </div>
+                    
+                    <div class="mt-4">
+                        <a href="{% url 'stiftung:kalender' %}" class="btn btn-secondary">
+                            <i class="fas fa-arrow-left me-1"></i>Zurück zum Kalender
+                        </a>
+                    </div>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>
+{% endblock %}