feat: Add organization name and scheduler refresh interval settings

- Superadmin-only organization name setting displayed in dashboard header
- Advanced Options tab with configurable scheduler refresh interval (0 = disabled)
- Make system settings GET endpoint public for frontend reads
- Scheduler reads refresh_seconds from DB dynamically each loop
- Seed default system settings in init_defaults.py

server/init_defaults.py

@@ -66,6 +66,8 @@ with engine.connect() as conn:
             ('video_loop', 'true', 'Loop (Wiederholung) für Videos'),
             ('video_volume', '0.8', 'Standard Lautstärke für Videos (0.0 - 1.0)'),
             ('holiday_banner_enabled', 'true', 'Ferienstatus-Banner auf Dashboard anzeigen'),
+            ('organization_name', '', 'Name der Organisation (wird im Header angezeigt)'),
+            ('refresh_seconds', '0', 'Scheduler Republish-Intervall (Sekunden; 0 deaktiviert)'),
         ]
     
         for key, value, description in default_settings: