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/models.py

@@ -2864,7 +2864,11 @@ class GeschichteSeite(models.Model):
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
     titel = models.CharField(max_length=200, verbose_name="Titel")
     slug = models.SlugField(max_length=200, unique=True, verbose_name="URL-Slug")
-    inhalt = models.TextField(verbose_name="Inhalt")
+    inhalt = models.TextField(
+        verbose_name="Inhalt (Markdown)", 
+        blank=True,
+        help_text="Sie können Markdown verwenden: **fett**, *kursiv*, # Überschriften, [Links](URL), Listen, etc."
+    )
     
     # Metadata
     erstellt_am = models.DateTimeField(auto_now_add=True, verbose_name="Erstellt am")
@@ -2940,3 +2944,124 @@ class GeschichteBild(models.Model):
     
     def __str__(self):
         return f"{self.titel} ({self.seite.titel})"
+
+
+class StiftungsKalenderEintrag(models.Model):
+    """Custom calendar events for foundation management"""
+    
+    KATEGORIE_CHOICES = [
+        ('termin', 'Termin/Meeting'),
+        ('zahlung', 'Zahlungserinnerung'),
+        ('deadline', 'Frist/Deadline'),
+        ('geburtstag', 'Geburtstag'),
+        ('vertrag', 'Vertrag läuft aus'),
+        ('pruefung', 'Prüfung/Nachweis'),
+        ('sonstiges', 'Sonstiges'),
+    ]
+    
+    PRIORITAET_CHOICES = [
+        ('niedrig', 'Niedrig'),
+        ('normal', 'Normal'),
+        ('hoch', 'Hoch'),
+        ('kritisch', 'Kritisch'),
+    ]
+    
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    titel = models.CharField(max_length=200, verbose_name="Titel")
+    beschreibung = models.TextField(blank=True, verbose_name="Beschreibung")
+    
+    # Date and time
+    datum = models.DateField(verbose_name="Datum")
+    uhrzeit = models.TimeField(null=True, blank=True, verbose_name="Uhrzeit")
+    ganztags = models.BooleanField(default=True, verbose_name="Ganztägig")
+    
+    # Categorization
+    kategorie = models.CharField(
+        max_length=20, 
+        choices=KATEGORIE_CHOICES, 
+        default='termin',
+        verbose_name="Kategorie"
+    )
+    prioritaet = models.CharField(
+        max_length=20,
+        choices=PRIORITAET_CHOICES,
+        default='normal',
+        verbose_name="Priorität"
+    )
+    
+    # Links to related objects
+    destinataer = models.ForeignKey(
+        'Destinataer',
+        null=True,
+        blank=True,
+        on_delete=models.CASCADE,
+        verbose_name="Bezogener Destinatär"
+    )
+    verpachtung = models.ForeignKey(
+        'LandVerpachtung',
+        null=True,
+        blank=True,
+        on_delete=models.CASCADE,
+        verbose_name="Bezogene Verpachtung"
+    )
+    
+    # Status and completion
+    erledigt = models.BooleanField(default=False, verbose_name="Erledigt")
+    erledigt_am = models.DateTimeField(null=True, blank=True, verbose_name="Erledigt am")
+    
+    # Metadata
+    erstellt_von = models.CharField(
+        max_length=100,
+        null=True,
+        blank=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")
+    
+    class Meta:
+        verbose_name = "Kalender Eintrag"
+        verbose_name_plural = "Kalender Einträge"
+        ordering = ['datum', 'uhrzeit']
+        indexes = [
+            models.Index(fields=['datum']),
+            models.Index(fields=['kategorie', 'datum']),
+            models.Index(fields=['erledigt', 'datum']),
+        ]
+    
+    def __str__(self):
+        return f"{self.datum}: {self.titel}"
+    
+    def get_kategorie_icon(self):
+        icons = {
+            'termin': 'fas fa-calendar-alt',
+            'zahlung': 'fas fa-euro-sign',
+            'deadline': 'fas fa-exclamation-triangle',
+            'geburtstag': 'fas fa-birthday-cake',
+            'vertrag': 'fas fa-file-contract',
+            'pruefung': 'fas fa-clipboard-check',
+            'sonstiges': 'fas fa-calendar',
+        }
+        return icons.get(self.kategorie, 'fas fa-calendar')
+    
+    def get_prioritaet_color(self):
+        colors = {
+            'niedrig': 'success',
+            'normal': 'primary',
+            'hoch': 'warning',
+            'kritisch': 'danger',
+        }
+        return colors.get(self.prioritaet, 'primary')
+    
+    def is_overdue(self):
+        """Check if event is overdue (past due and not completed)"""
+        if self.erledigt:
+            return False
+        return self.datum < timezone.now().date()
+    
+    def is_upcoming(self, days=7):
+        """Check if event is upcoming within specified days"""
+        if self.erledigt:
+            return False
+        today = timezone.now().date()
+        return today <= self.datum <= (today + timezone.timedelta(days=days))