Add schedule start lifecycle locking

[jjoonleo]

Summary

• Add server-owned schedule lifecycle timestamps with startedAt and finishedAt migrations.
• Add idempotent POST /schedules/{scheduleId}/start that freezes preparation snapshots and returns schedule plus preparations.
• Guard schedule edits, schedule-specific preparation edits, deletion, and finish with explicit lifecycle rules.
• Refactor punctuality scoring to count only explicitly finished NORMAL and LATE schedules.
• Document the frontend API changes in docs/schedule-start-api.md.

Frontend impact

• Schedule responses include nullable startedAt and finishedAt.
• Alarm window responses include nullable startedAt and finishedAt.
• Finish now rejects unstarted schedules with SCHEDULE_NOT_STARTED.
• Started schedules use frozen schedule-specific preparation snapshots, so default preparation edits do not affect running schedules.

Validation

• JAVA_HOME="$HOME/.sdkman/candidates/java/17.0.12-oracle" ./gradlew test