Sync engine: push + pull + last-write-wins

[fboucher]

Parent PRD

#110

Depends on

#125 (offline write queue), #121 (delta API endpoints with modifiedAfter)

Summary

Implement SyncService — triggered on App.OnResume and Connectivity.ConnectivityChanged (online transition). Push queued local changes to the API, then pull any remote changes since last sync. Apply last-write-wins conflict resolution using DateModified.

SyncService — push phase

1. Query all local records where IsPendingSync = true
2. PATCH/POST each to the API
3. On success, clear IsPendingSync = false
4. Soft-deleted records pushed as deletes, then removed locally on success

SyncService — pull phase

1. Call GET /api/posts?modifiedAfter={LastSyncTimestamp}
2. Call GET /api/notes?modifiedAfter={LastSyncTimestamp}
3. For each returned record, compare DateModified against local copy
4. Keep whichever has the later DateModified (last-write-wins)
5. Update LastSyncTimestamp = DateTime.UtcNow in Preferences after successful sync

Trigger points

• App.OnResume (foreground sync when resuming app)
• Connectivity.ConnectivityChanged event when transitioning to online

Tests (unit — mock dependencies)

• Push phase: sends all IsPendingSync=true records; clears flag on success
• Pull phase: remote newer → remote wins; local newer → local wins
• LastSyncTimestamp updated after successful full sync
• Use xUnit + FluentAssertions + Moq (already in codebase)

Acceptance Criteria

• SyncService implemented with push and pull phases
• Triggered on app resume and connectivity restored
• Last-write-wins applied correctly on both sides
• LastSyncTimestamp persisted in Preferences
• Unit tests pass for push, pull, and conflict resolution

Stories from PRD

User stories 7, 8, 17, 18, 19