Add UserRepository.dashboard_row for single-row refresh

Extracts the dashboard_rows() loop body into a shared
_build_dashboard_row helper so a single rider's row can be re-queried
after a sync action, without changing dashboard_rows()'s behavior.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/db/test_repositories.py

@@ -207,3 +207,19 @@ def test_dashboard_summary_success_rate_is_none_without_finished_runs_in_window(
     summary = user_repository.dashboard_summary(since=datetime.now(timezone.utc) - timedelta(days=7))
 
     assert summary.success_rate_recent is None
+
+
+def test_dashboard_row_returns_row_for_known_user(user_repository) -> None:
+    user = _make_user(user_repository, "Alex")
+
+    row = user_repository.dashboard_row(user.id)
+
+    assert row is not None
+    assert row.id == user.id
+    assert row.name == "Alex"
+
+
+def test_dashboard_row_returns_none_for_unknown_user(user_repository) -> None:
+    row = user_repository.dashboard_row(999)
+
+    assert row is None