Record queue admission and operator stops (DRU-507)

[czpython]

Record each newly accepted subject-bound app run before the request transaction commits. Record successful operator stops with the reason and exact run. Repeated cancellation returns already_cancelled without another record.

Decisions

• Use the existing workflow.scheduled and workflow.cancelled topics for admission and operator stops.
• Let the existing event recorder accept the separate admission session, so it preserves the caller's transaction.