Choreographer takes an audit dep but never used it. Now every rejection
Envelope (invalid_state, not_authorized, tracing_gap, not_found) writes a
gateway.rejected row with verb + reason + missing fields. Forensic
signal for stuck flows; no behaviour change for happy path.
Adds AuditService.log_event() — generic free-form event_type write so
the gateway doesn't have to extend AuditEventType for every new surface.
Audit writes are best-effort: a failure logs a warning but never
propagates, since the agent's response Envelope is the contract.
Renn F
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