The poller's Start() calls GetMaxTransmissionID() to initialize its cursor.
When the test goroutine inserts data between go poller.Start() and the
actual GetMaxTransmissionID() call, the poller's cursor skips past the
test data and never broadcasts it, causing a timeout.
Adding a 100ms sleep after go poller.Start() ensures the poller has
initialized its cursors before the test inserts new data.
IngestNewFromDB now broadcasts one message per observation (not per
transmission). IngestNewObservations also broadcasts late arrivals.
Tests verify multi-observer packets produce multiple WS messages.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>