fix: unify alarm fallback permission policy

[jjoonleo]

Summary

• add a shared alarm delivery policy for native, fallback, permission-blocked, and unsupported states
• use the shared policy from AlarmGateCubit, My Page, and ReconcileAlarmsUseCase
• document the fallback-enabled delivery decision in ADR-0010

Closes #534

Tests

• flutter test test/domain/entities/alarm_delivery_policy_test.dart test/presentation/app/cubit/alarm_gate_cubit_test.dart test/presentation/my_page/my_page_screen_test.dart test/domain/use-cases/reconcile_alarms_use_case_test.dart
• flutter analyze