fix(ri-exchange): exchange records never carry a creator, so four-eyes is a no-op and audit attribution is lost #518

Summary

RI exchange records never carry a creator, so the four-eyes check from #501 is a no-op in production and approver attribution is incomplete.

Location (origin/main 496d9d7)

• internal/server/handler_ri_exchange.go:329 exchangeToConfigRecord copies every field except CreatedByUserID, so every production exchange row has a NULL creator.
• internal/api/handler_ri_exchange.go:2390 requireDifferentRIExchangeApprover returns nil when the creator is NULL, so the check is skipped for all rows. Owner policy for creator-less rows is open in fix(purchase): scheduler-created executions have no creator and cannot pass four-eyes #233.
• internal/api/handler_ri_exchange.go:2220-2222 token approval calls TransitionRIExchangeStatus(..., nil), so transitioned_by is NULL. StampRIExchangeApprovedBy is only called on the session path (~2281), so approved_by is never set for token approvals even when a session resolves.
• Wrong comments: internal/api/handler_ri_exchange.go:2220 ("no session user"), internal/config/types.go:963-965 ("populated for dashboard-initiated exchanges" although no such writer exists).

Failure scenario

A user creates an exchange, then approves it themselves. Four-eyes does not fire because the stored creator is NULL. Audit rows show no actor for token approvals.

Context

Found in the #501 review. Related: #233, #82.

Acceptance

Exchange rows created by a session user store created_by_user_id, four-eyes rejects a self-approval in a test using the real adapter, and token approvals record the resolved actor when one exists.