Set an organization's capability overrides

Enables, disables or clears (null) per-organization feature overrides. Every key in the feature registry (packages/features/src/registry.ts) is accepted; a feature that is fixed on this deployment (unavailable, off or on for everyone) returns 400, and unknown or retired keys are ignored. A deployment lock (DEN_FEATURE_*) outranks a stored override; featureStates shows the effective value and its source. The deprecated gatewayDashboard boolean or null input is validated but ignored and never persisted; its response field is always true.