Operator acceptance of a retire-or-retune proposal (Phase 200-02).
The approval_action.proposal_id emitted in the morning brief’s
roi_deep_dive.unattributed_decisions[] section is the exact UUID
this endpoint expects. On success the proposal is marked accepted
and an :attr:MessageType.ATTRIBUTION_PROPOSAL_ACCEPTED event is
published on the message bus (bus failure is swallowed by the
service so transient bus outages never fail the operator’s click).
Tenant-isolated via :class:UserContext. Returns 404 (not 403) when
the proposal does not exist, belongs to a different tenant, or was
already accepted – matches the Phase 199-03 “leak zero bits about
other tenants’ namespaces” convention.