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/stiftung/migrations/0040_add_calendar_model.py

@@ -0,0 +1,13 @@
+# Generated by Django 5.0.6 on 2025-10-04 20:28
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0039_stiftungskalendereintrag'),
+    ]
+
+    operations = [
+    ]