Implement Way Forward Task Service

Implement idempotency using a composite unique constraint in Supabase on (report_id, item_index) — the database enforces uniqueness at the insert level, avoiding race conditions from concurrent retries. In the service, use an upsert with onConflict: DoNothing and check affected rows to distinguish insert vs. already-exists. Wrap the full persist + task-queue signal in a Supabase database transaction or a logical transaction in Dart (persist all items first, then signal queue only on full success).

Define a WayForwardTaskCreationResult sealed class (created / alreadyExisted / failed) to give callers precise feedback. Due date calculation should be extracted into a pure function (daysToAdd: int, from: DateTime) → DateTime for easy unit testing. Note: this service is invoked synchronously within the report submission flow — keep it fast. If task queue signalling is slow, consider fire-and-forget with unawaited().

Unit tests with flutter_test and mockito covering: (1) successful creation of one coordinator task per way-forward item, (2) idempotency — second call with same report_id returns existing tasks without inserting, (3) repository failure causes full rollback with no partial records, (4) task queue signal is emitted exactly once per new task, (5) due date calculated correctly from submission date + configured offset, (6) coordinator assignment falls back to org default when report has no coordinator_id. Mock way-forward-item-repository and task queue. Validate that no real Supabase calls are made in unit tests. Target 90%+ line coverage.