Implement Coins shared group cloud sync backend

[ucguy4u]

Problem

Coins now has an app-side Local / Cloud mode gate for shared groups, but the actual cloud synchronization backend is not implemented yet. Group invites can be generated and parsed in the app, but joining across devices/peers still needs backend invite validation, membership creation, and event-log sync.

Scope

In scope:

• Add Firebase Auth-backed group membership for Google users.
• Add Firestore data model for shared groups, members, invites, events, expenses, splits, and settlements.
• Add short-lived invite token creation/validation for https://airo.app/coins/join links.
• Add offline-first group sync: local outbox, immutable event log, replay into local SQLite, listener-based updates.
• Add Firestore security rules so only group members can read/write group data.
• Keep personal transactions, personal budgets, private receipts, and local AI finance insights local by default.

Out of scope:

• Syncing the personal ledger by default.
• Migrating all Coins storage to cloud.
• Firebase Dynamic Links, because that product is deprecated.

Acceptance criteria

• Local mode works without Google sign-in.
• Cloud mode requires Google sign-in before invite creation or shared group sync.
• Owner can create a cloud invite and share an Airo-owned HTTPS link.
• Receiver can open/paste the link, sign in with Google, and join the group.
• Shared group expenses, splits, members, and settlements sync across at least two signed-in users/devices.
• Offline writes queue locally and replay after reconnect without losing group state.
• Sync replay from the cloud event log into local SQLite is deterministic.
• Firestore rules deny reads/writes for non-members.
• Unit/integration tests cover invite parsing, membership rules, event replay, and conflict handling.
• Docs update references docs/superpowers/plans/2026-06-22-coins-local-cloud-sharing.md.

Agent

Primary: agent/offline-sync
Supporting: agent/finance, agent/security

Estimate

8-13 points, likely split into backend schema/rules, app sync client, and QA follow-up cards if it grows.

Risks

• Data privacy: personal money data must not upload unless explicitly shared.
• Security: invite tokens must be scoped, short-lived, and non-enumerable.
• Conflict handling: split and settlement conflicts need review-required states, not silent overwrites.
• Rollout: existing local groups need a clear upgrade path before cloud sharing is enabled for them.

Context

App-side precursor merged locally in main:

• feat(coins): add local cloud sharing mode
• feat(coins): merge local cloud sharing mode

[ucguy4u]

Marking this as human/setup blocked before full implementation can be completed or verified.

Required human/account decisions:

• Confirm the Firebase project/environment that should host Coins shared-group sync.
• Confirm Google sign-in OAuth client setup for the target app package IDs and environments.
• Confirm Firestore region, billing/quotas, retention expectations, and backup/export policy.
• Provide or authorize deployment credentials for Firestore rules/indexes and any backend functions if this becomes server-backed.
• Confirm whether shared Coins cloud sync is in scope for the current v2 release wave or should remain deferred behind the Local / Cloud mode gate.

Engineering can still draft schemas/rules/client code, but the acceptance criteria requiring real cross-device sync, invite validation, and member-only Firestore access cannot be truthfully closed without those Firebase resources and credentials.

[ucguy4u]

Host-side v2 refresh on 2026-07-15:

• cd app && flutter test test/features/coins/application/services/coins_invite_link_service_test.dart test/features/coins/application/use_cases/add_group_member_use_case_test.dart test/features/coins/application/providers/create_group_provider_test.dart test/features/coins/data/repositories/group_repository_impl_test.dart test/features/coins/presentation/screens/group_detail_screen_test.dart ✅ 39/39 passed after clearing stale generated native assets with rm -rf app/build/native_assets.
• cd packages/core_data && flutter test test/sync/sync_operation_test.dart test/sync/sync_service_test.dart ✅ 15/15 passed.

This confirms local invite-link parsing, group/member repository behavior, group UI debt display, and generic outbox/sync contracts are healthy. The issue remains blocked for closure because production cloud sync still requires Firebase/Auth/Firestore project decisions, Google sign-in/OAuth setup, rules/index deployment access, real member-only Firestore security verification, and cross-device sync evidence. No CI build, release build, publishing, app distribution, or monitoring action was triggered.