Add active scheduling of "Station Fallback" in form of a default show

Parent: [EPIC] Proactive scheduling of fallback program (aura#392)

• Actively schedule Station Fallback when no planned timeslots are expected, to avoid wait time for silence detector
• Handle like a Default Station Playlist, similar to Default Playlists on schedule and show

Dependencies

• [EPIC] Basic radio station administration (aura#221 - closed)
• [EPIC] Display virtual timeslots / Music Fallba... (aura#176 - closed)