Trigger background sync on connectivity recovery

docs/progress.md

@@ -2,6 +2,34 @@
 
 Updated: 2026-02-17
 
+## Latest Milestone (2026-02-17): Connectivity Event-Driven Background Sync
+
+Implemented online-transition-triggered sync so background sync reacts faster
+when network returns:
+
+- Added connectivity stream support to reachability abstraction:
+  - `lib/core/network/reachability/network_reachability.dart`
+  - new `watch(...)` API for reachability change events
+- Added IO implementation using connectivity plugin + internet probe:
+  - `lib/core/network/reachability/network_reachability_io.dart`
+  - `connectivity_plus` integrated as transport signal source
+- Updated background runner:
+  - `lib/features/sync/sync_background_runner.dart`
+  - now listens for offline -> online transitions and triggers immediate sync
+  - keeps existing startup/resume/periodic behavior
+- Added coverage:
+  - `test/features/sync/sync_background_runner_test.dart`
+  - verifies transition-trigger behavior and avoids duplicate triggers when
+    already online
+- Updated setup docs:
+  - `docs/SETUP.md` background sync section now includes connectivity-return
+    behavior note
+
+Validation for this milestone:
+
+- `flutter analyze` -> pass
+- `flutter test` -> pass
+
 ## Latest Milestone (2026-02-17): Sync Repair Test Stability + Mobile CRUD/Sync Flow Coverage
 
 Completed two test-depth upgrades:
@@ -313,8 +341,8 @@ Validation for this milestone:
    - future enhancement: auto-select and highlight the exact row in main feature
      screens after save.
 3. Sync trigger maturity:
-   - implemented via reachability gating; future improvement is explicit OS
-     connectivity event subscription.
+   - implemented via reachability gating + online-transition event triggers.
+   - future: add richer platform-specific network quality awareness if needed.
 4. Test depth:
    - expanded with mobile compact CRUD+sync flow and rejected-repair flow.
    - future: add desktop + web-specific end-to-end sync error/retry scenario.