[codex] Transition preparation API contract

[jjoonleo]

Summary

• Add preparation template API models, repository, data source, and use cases.
• Add ordered preparation step mapping while preserving existing linked-list preparation entities internally.
• Update schedule create/update request models and schedule/alarm-window response parsing for preparation source metadata and frozen schedules.
• Keep existing schedule update flows preserving preparation source unless explicitly requested.

Validation

• dart run build_runner build --delete-conflicting-outputs
• flutter analyze
• flutter test

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