feat(security): harden gateway boundaries and usage policies

Consolidate subscription usage policy enforcement, privacy-safe persistence, and gateway security hardening into one reviewable change.

Includes bounded HTTP and execution envelopes, header and protocol guards, DNS and relay validation, authentication and secret projection hardening, secure backup/install paths, and regression coverage.

apps/aether-gateway/src/audit/http.rs

@@ -26,7 +26,10 @@ pub(crate) async fn get_request_candidate_trace(
         .map_err(|err| GatewayError::Internal(err.to_string()).into_response())?;
 
     match trace {
-        Some(trace) => Ok(Json(trace)),
+        Some(mut trace) => {
+            trace.sanitize_sensitive_diagnostics();
+            Ok(Json(trace))
+        }
         None => Err((
             axum::http::StatusCode::NOT_FOUND,
             Json(json!({
@@ -52,7 +55,10 @@ pub(crate) async fn get_decision_trace(
         .map_err(|err| GatewayError::Internal(err.to_string()).into_response())?;
 
     match trace {
-        Some(trace) => Ok(Json(trace)),
+        Some(mut trace) => {
+            trace.sanitize_sensitive_diagnostics();
+            Ok(Json(trace))
+        }
         None => Err((
             axum::http::StatusCode::NOT_FOUND,
             Json(json!({