BE003: map Firebase-SQLite sync coverage

[shshwtsuthar]

Summary

Adds docs/backend/offline-sync.md, an evidence-backed map of the Firebase Firestore and SQLite boundary.

It classifies profiles, shopping lists/items, favourites, history, and product catalogue snapshots; documents direction, triggers, conflicts, retry behaviour, unknowns, and follow-up tickets. No sync logic or schemas were changed.

Validation

• git diff --check passed.
• npm test -- --runInBand tests/profileSync.test.ts ran: 18 passed, 3 failed in the unmodified existing suite. One test exceeds Jests default timeout because the implemented retry backoff totals 7 seconds; two assertions expect writes that the current hasChanged guard intentionally skips. These are documented as a follow-up; no test or production source was changed in this documentation-only PR.