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/wsgi.py

@@ -2,6 +2,7 @@
 from server.routes.eventmedia import eventmedia_bp
 from server.routes.files import files_bp
 from server.routes.events import events_bp
+from server.routes.holidays import holidays_bp
 from server.routes.groups import groups_bp
 from server.routes.clients import clients_bp
 from server.database import Session, engine
@@ -20,6 +21,7 @@ app.register_blueprint(groups_bp)
 app.register_blueprint(events_bp)
 app.register_blueprint(eventmedia_bp)
 app.register_blueprint(files_bp)
+app.register_blueprint(holidays_bp)
 
 
 @app.route("/health")