fix: allow scheduled Apple Health medication mirrors

[muhdusama]

Summary

Allow Apple Health medication mirrors with asNeeded: false to be created without requiring a HealthLog-owned schedule.

The iOS client maps HealthKit medications that have an Apple schedule to asNeeded: false, but does not send HealthLog schedules. The server previously rejected those mirrors with HTTP 422 (Mindestens ein Zeitfenster).

This change exempts externalSource: APPLE_HEALTH from the local schedule requirement while preserving the existing validation for native HealthLog medications.

Tests

• src/app/api/medications/__tests__/route.test.ts: 33/33 passed on current main
• Added regression coverage for a scheduled Apple Health mirror without a HealthLog-owned schedule
• End-to-end sync validation confirmed the previously rejected request shape now returns HTTP 201 and remains as_needed=false