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/0038_allow_blank_content.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2025-10-02 20:07
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0037_add_geschichte_models'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='geschichteseite',
+            name='inhalt',
+            field=models.TextField(blank=True, verbose_name='Inhalt'),
+        ),
+    ]

app/stiftung/migrations/0039_stiftungskalendereintrag.py

@@ -0,0 +1,41 @@
+# Generated by Django 5.0.6 on 2025-10-04 20:21
+
+import django.db.models.deletion
+import uuid
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0038_allow_blank_content'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='StiftungsKalenderEintrag',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('titel', models.CharField(max_length=200, verbose_name='Titel')),
+                ('beschreibung', models.TextField(blank=True, verbose_name='Beschreibung')),
+                ('datum', models.DateField(verbose_name='Datum')),
+                ('uhrzeit', models.TimeField(blank=True, null=True, verbose_name='Uhrzeit')),
+                ('ganztags', models.BooleanField(default=True, verbose_name='Ganztägig')),
+                ('kategorie', models.CharField(choices=[('termin', 'Termin/Meeting'), ('zahlung', 'Zahlungserinnerung'), ('deadline', 'Frist/Deadline'), ('geburtstag', 'Geburtstag'), ('vertrag', 'Vertrag läuft aus'), ('pruefung', 'Prüfung/Nachweis'), ('sonstiges', 'Sonstiges')], default='termin', max_length=20, verbose_name='Kategorie')),
+                ('prioritaet', models.CharField(choices=[('niedrig', 'Niedrig'), ('normal', 'Normal'), ('hoch', 'Hoch'), ('kritisch', 'Kritisch')], default='normal', max_length=20, verbose_name='Priorität')),
+                ('erledigt', models.BooleanField(default=False, verbose_name='Erledigt')),
+                ('erledigt_am', models.DateTimeField(blank=True, null=True, verbose_name='Erledigt am')),
+                ('erstellt_von', models.CharField(blank=True, max_length=100, null=True, verbose_name='Erstellt von')),
+                ('erstellt_am', models.DateTimeField(auto_now_add=True, verbose_name='Erstellt am')),
+                ('aktualisiert_am', models.DateTimeField(auto_now=True, verbose_name='Aktualisiert am')),
+                ('destinataer', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, to='stiftung.destinataer', verbose_name='Bezogener Destinatär')),
+                ('verpachtung', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, to='stiftung.landverpachtung', verbose_name='Bezogene Verpachtung')),
+            ],
+            options={
+                'verbose_name': 'Kalender Eintrag',
+                'verbose_name_plural': 'Kalender Einträge',
+                'ordering': ['datum', 'uhrzeit'],
+                'indexes': [models.Index(fields=['datum'], name='stiftung_st_datum_9e97ed_idx'), models.Index(fields=['kategorie', 'datum'], name='stiftung_st_kategor_d7c7f9_idx'), models.Index(fields=['erledigt', 'datum'], name='stiftung_st_erledig_115235_idx')],
+            },
+        ),
+    ]

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 = [
+    ]

app/stiftung/migrations/0041_alter_geschichteseite_inhalt.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2025-10-04 21:49
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0040_add_calendar_model'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='geschichteseite',
+            name='inhalt',
+            field=models.TextField(blank=True, help_text='Sie können Markdown verwenden: **fett**, *kursiv*, # Überschriften, [Links](URL), Listen, etc.', verbose_name='Inhalt (Markdown)'),
+        ),
+    ]