[dist] Correlation severed at async boundary

[Aparnap2]

From distributed-systems audit. correlation_id never enters outbox payload or Pub/Sub attrs (forced triple event_id/event_type/tenant_id); push handler drops Fiber locals; observe() cannot emit them. Fix: add correlation_id to BuildUploadPayload + publish attrs, bridge in DocumentPushHandler/DocumentOutcomeHandler, log in observe(). Critical for reconstructing claim->document->exception->investigation chains. Priority: High.