Data initialization watchdog is fully reactive

A hub whose data sources hang or fail during startup is now settled by a subscription to the actual completion signal rather than a background timer race. The behaviour a user sees is unchanged in the good cases and sharper in the bad ones: a hung initialization still trips the same time-box and answers requests immediately with a clear "did not complete within…" diagnostic, a failed initialization reports the specific error, and a page closed mid-startup no longer risks a stray timeout being logged minutes later.