Adds engine.runDueWaits() — polls waiting runs whose wake_at has passed and
resumes the ones parked on a wait node (gate timeouts handled later by the
approvals layer). Flag-gated (fails closed when workflows is off). Wired into
the existing hourly invoiceScheduler tick in its own try/catch so a workflow
failure never suppresses the invoice/reminder jobs. Test covers not-due vs
elapsed resume.
Graph executor that walks nodes/edges per run: trigger, condition/branch
(registered conditions → yes/no edge), bounded loop (counter in context +
maxIterations cap), wait (status=waiting + wake_at for the scheduler), gate
(status=waiting; resumed via confirm/deny edge), action/webhook (registered
handlers). emitWorkflowEvent creates one idempotent run per matching enabled
workflow (unique dedup_key) and fails CLOSED if the flag system is
unavailable; never throws into callers (safe to call after commit). Every
node records a workflow_run_steps row. Registry seeds primitive
conditions (always/never/expr) + actions (noop/log/set_context). Integration
test covers loop+wait resume, gate confirm, and dedup.
Luca