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/alembic/versions/8d1df7199cb7_add_academic_periods_system.py

@@ -0,0 +1,62 @@
+"""add academic periods system
+
+Revision ID: 8d1df7199cb7
+Revises: 71ba7ab08d84
+Create Date: 2025-09-20 11:07:08.059374
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '8d1df7199cb7'
+down_revision: Union[str, None] = '71ba7ab08d84'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('academic_periods',
+    sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+    sa.Column('name', sa.String(length=100), nullable=False),
+    sa.Column('display_name', sa.String(length=50), nullable=True),
+    sa.Column('start_date', sa.Date(), nullable=False),
+    sa.Column('end_date', sa.Date(), nullable=False),
+    sa.Column('period_type', sa.Enum('schuljahr', 'semester', 'trimester', name='academicperiodtype'), nullable=False),
+    sa.Column('is_active', sa.Boolean(), nullable=False),
+    sa.Column('created_at', sa.TIMESTAMP(timezone=True), server_default=sa.text('CURRENT_TIMESTAMP'), nullable=True),
+    sa.Column('updated_at', sa.TIMESTAMP(timezone=True), server_default=sa.text('CURRENT_TIMESTAMP'), nullable=True),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('name', name='uq_academic_periods_name')
+    )
+    op.create_index('ix_academic_periods_active', 'academic_periods', ['is_active'], unique=False)
+    op.create_index(op.f('ix_academic_periods_end_date'), 'academic_periods', ['end_date'], unique=False)
+    op.create_index(op.f('ix_academic_periods_start_date'), 'academic_periods', ['start_date'], unique=False)
+    op.add_column('event_media', sa.Column('academic_period_id', sa.Integer(), nullable=True))
+    op.create_index(op.f('ix_event_media_academic_period_id'), 'event_media', ['academic_period_id'], unique=False)
+    op.create_foreign_key(None, 'event_media', 'academic_periods', ['academic_period_id'], ['id'])
+    op.add_column('events', sa.Column('academic_period_id', sa.Integer(), nullable=True))
+    op.create_index(op.f('ix_events_academic_period_id'), 'events', ['academic_period_id'], unique=False)
+    op.create_foreign_key(None, 'events', 'academic_periods', ['academic_period_id'], ['id'])
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_constraint(None, 'events', type_='foreignkey')
+    op.drop_index(op.f('ix_events_academic_period_id'), table_name='events')
+    op.drop_column('events', 'academic_period_id')
+    op.drop_constraint(None, 'event_media', type_='foreignkey')
+    op.drop_index(op.f('ix_event_media_academic_period_id'), table_name='event_media')
+    op.drop_column('event_media', 'academic_period_id')
+    op.drop_index(op.f('ix_academic_periods_start_date'), table_name='academic_periods')
+    op.drop_index(op.f('ix_academic_periods_end_date'), table_name='academic_periods')
+    op.drop_index('ix_academic_periods_active', table_name='academic_periods')
+    op.drop_table('academic_periods')
+    # ### end Alembic commands ###