Define food ledger storage and backup protocol

[syamaner]

Summary

• select a serialized SQLite/GRDB local ledger with explicit immutable versions and append-only operations
• define deterministic backup generations, staged restore, idempotent retry and conflict-preserving merge
• define protected local attachments, side-by-side datasets, corruption/deletion behaviour and future Drive boundaries
• record the completed UX re-triage and keep Phase container: Local-first food identity logging #94 as a phase container awaiting decomposition

Validation

• validated local Markdown links
• checked trailing whitespace
• git diff --check

Planning only: no application code, package dependency, OAuth, Drive, provider, device or HealthKit change.

Closes #90

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!