docs/dev: sync backend rework, MQTT, and devcontainer hygiene

README: add Versioning (unified SemVer, pre-releases, build metadata); emphasize UTC handling and streaming endpoint; add Dev Container notes (UI-only Remote Containers, npm ci, idempotent aliases)
TECH-CHANGELOG: backend rework notes (serialization camelCase, UTC normalization, streaming metadata); add component build metadata template (image tags/SHAs)
Copilot instructions: integrate maintenance guardrails; reinforce UTC and camelCase conventions; document MQTT topics and scheduler retained payload behavior
Devcontainer: map Remote Containers to UI; remove in-container install; switch to npm ci; make aliases idempotent

server/routes/system_settings.py

@@ -202,3 +202,66 @@ def update_supplement_table_settings():
     finally:
         session.close()
 
+
+@system_settings_bp.route('/holiday-banner', methods=['GET'])
+def get_holiday_banner_setting():
+    """
+    Get holiday banner enabled status.
+    Public endpoint - dashboard needs this.
+    """
+    session = Session()
+    try:
+        setting = session.query(SystemSetting).filter_by(key='holiday_banner_enabled').first()
+        enabled = setting.value == 'true' if setting else True
+        
+        return jsonify({'enabled': enabled}), 200
+    except SQLAlchemyError as e:
+        return jsonify({'error': str(e)}), 500
+    finally:
+        session.close()
+
+
+@system_settings_bp.route('/holiday-banner', methods=['POST'])
+@admin_or_higher
+def update_holiday_banner_setting():
+    """
+    Update holiday banner enabled status.
+    Admin+ only.
+    
+    Request body:
+    {
+        "enabled": true/false
+    }
+    """
+    session = Session()
+    try:
+        data = request.get_json()
+        if not data:
+            return jsonify({'error': 'No data provided'}), 400
+        
+        enabled = data.get('enabled', True)
+        
+        # Update or create setting
+        setting = session.query(SystemSetting).filter_by(key='holiday_banner_enabled').first()
+        if setting:
+            setting.value = 'true' if enabled else 'false'
+        else:
+            setting = SystemSetting(
+                key='holiday_banner_enabled',
+                value='true' if enabled else 'false',
+                description='Ferienstatus-Banner auf Dashboard anzeigen'
+            )
+            session.add(setting)
+        
+        session.commit()
+        
+        return jsonify({
+            'enabled': enabled,
+            'message': 'Holiday banner setting updated successfully'
+        }), 200
+    except SQLAlchemyError as e:
+        session.rollback()
+        return jsonify({'error': str(e)}), 500
+    finally:
+        session.close()
+