feat: Add academic periods system for educational institutions

- Add AcademicPeriod model with support for schuljahr/semester/trimester
- Extend Event and EventMedia models with optional academic_period_id
- Create Alembic migration (8d1df7199cb7) for academic periods system
- Add init script for Austrian school year defaults (2024/25-2026/27)
- Maintain full backward compatibility for existing events/media
- Update program-info.json to version 2025.1.0-alpha.6

Database changes:
- New academic_periods table with unique name constraint
- Foreign key relationships with proper indexing
- Support for multiple period types with single active period

This lays the foundation for period-based organization of events
and media content, specifically designed for school environments
with future extensibility for universities.

server/init_academic_periods.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+"""
+Erstellt Standard-Schuljahre für österreichische Schulen
+Führe dieses Skript nach der Migration aus, um Standard-Perioden zu erstellen.
+"""
+
+from datetime import date
+from models.models import AcademicPeriod, AcademicPeriodType
+from server.database import Session
+import sys
+sys.path.append('/workspace')
+
+
+def create_default_academic_periods():
+    """Erstellt Standard-Schuljahre für österreichische Schulen"""
+    session = Session()
+
+    try:
+        # Prüfe ob bereits Perioden existieren
+        existing = session.query(AcademicPeriod).first()
+        if existing:
+            print("Academic periods already exist. Skipping creation.")
+            return
+
+        # Standard Schuljahre erstellen
+        periods = [
+            {
+                'name': 'Schuljahr 2024/25',
+                'display_name': 'SJ 24/25',
+                'start_date': date(2024, 9, 2),
+                'end_date': date(2025, 7, 4),
+                'period_type': AcademicPeriodType.schuljahr,
+                'is_active': True  # Aktuelles Schuljahr
+            },
+            {
+                'name': 'Schuljahr 2025/26',
+                'display_name': 'SJ 25/26',
+                'start_date': date(2025, 9, 1),
+                'end_date': date(2026, 7, 3),
+                'period_type': AcademicPeriodType.schuljahr,
+                'is_active': False
+            },
+            {
+                'name': 'Schuljahr 2026/27',
+                'display_name': 'SJ 26/27',
+                'start_date': date(2026, 9, 7),
+                'end_date': date(2027, 7, 2),
+                'period_type': AcademicPeriodType.schuljahr,
+                'is_active': False
+            }
+        ]
+
+        for period_data in periods:
+            period = AcademicPeriod(**period_data)
+            session.add(period)
+
+        session.commit()
+        print(f"Successfully created {len(periods)} academic periods")
+
+        # Zeige erstellte Perioden
+        for period in session.query(AcademicPeriod).all():
+            status = "AKTIV" if period.is_active else "inaktiv"
+            print(
+                f"  - {period.name} ({period.start_date} - {period.end_date}) [{status}]")
+
+    except Exception as e:
+        session.rollback()
+        print(f"Error creating academic periods: {e}")
+    finally:
+        session.close()
+
+
+if __name__ == "__main__":
+    create_default_academic_periods()