[Core] Implement Geofencing Service

[sunpl13]

Overview

Implement the core background location monitoring logic using Geofencing.

Requirements

• Use geolocator or native platform channels to register region monitoring.
• Radius: Fixed at 100m (MVP).
• Triggers: Detect Enter events for registered locations.
• Battery Efficiency: Ensure the logic does not keep GPS on high accuracy continuously; rely on OS region monitoring.

References

• PRD Section 2.1 (Trigger Radius)
• PRD Section 3 (Battery Efficiency)