[codex] Implement backend alarm feature

[jjoonleo]

Summary

Implements the backend native alarm feature contract for account-owned schedules and single-device alarm reconciliation.

Changes

• Add user alarm settings, current device registration/unregistration, alarm status reporting, and current alarm diagnostics endpoints.
• Add alarm settings, user device, and latest alarm status persistence with migration defaults for existing users.
• Add /schedules/alarm-window with NOT_ENDED filtering, inclusive/exclusive range semantics, 14-day range validation, sorted output, preparation data, and computed debug alarm times.
• Link FCM tokens to the current active device and bind device metadata to the authenticated token session.
• Suppress legacy prep reminders only when a fresh current-device alarm status reports local coverage for the schedule.
• Preserve schedule/preparation API fields, sync doneStatus after schedule updates, and add default alarm settings for new social-auth users.
• Add focused alarm service tests and test-only H2 configuration so the suite can run without local MySQL.

Validation

• ./gradlew test
• ./gradlew test --tests devkor.ontime_back.service.AlarmServiceTest
• git diff --check