feat(video, settings, docs): add muted playback, nested Settings tabs, merge holidays tab; bump 2025.1.0-alpha.11

API/DB: add Event.muted with full CRUD wiring (Alembic migration), persist/return with autoplay/loop/volume
Dashboard: per‑event video options (autoplay/loop/volume/muted) with system defaults; Settings → Events → Videos defaults
Settings UX: nested tabs with controlled selection; Academic Calendar: merge “Schulferien Import”+“Liste” into “📥 Import & Liste”
Docs: update README and copilot-instructions (video payload, streaming 206, defaults keys); update program-info.json changelog; bump version to 2025.1.0‑alpha.11

server/alembic/versions/21226a449037_add_muted_to_events.py

@@ -0,0 +1,30 @@
+"""add_muted_to_events
+
+Revision ID: 21226a449037
+Revises: 910951fd300a
+Create Date: 2025-11-05 17:24:29.168692
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '21226a449037'
+down_revision: Union[str, None] = '910951fd300a'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # Add muted column to events table for video mute control
+    op.add_column('events', sa.Column('muted', sa.Boolean(), nullable=True))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # Remove muted column
+    op.drop_column('events', 'muted')