#792 Signals never wake a sleeping run (poll-only, asymmetric with events/approvals)

send_signal (engine/services/signal_service.py) inserts into workflow_signals; consumption is poll-based (check_and_consume_signal, FOR UPDATE SKIP LOCKED). A run suspended in wait_for_event cannot be advanced by a signal. Fix: emit a wake alongside the insert (mirror approval emit_event pattern), keep poll-consume semantics. Demo-gated.