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/ai_serving/planner/candidate_source.rs

@@ -1473,6 +1473,7 @@ mod tests {
     use super::*;
     use crate::data::GatewayDataState;
     use crate::AppState;
+    use aether_crypto::DEVELOPMENT_ENCRYPTION_KEY;
     use aether_data::repository::candidate_selection::InMemoryMinimalCandidateSelectionReadRepository;
     use aether_data::repository::provider_catalog::InMemoryProviderCatalogReadRepository;
     use aether_data::DataLayerError;
@@ -2174,6 +2175,19 @@ mod tests {
             None,
         )
         .expect("endpoint transport should build");
+        let credential_state = AppState::new()
+            .expect("credential state should build")
+            .with_data_state_for_tests(
+                GatewayDataState::disabled()
+                    .with_encryption_key_for_tests(DEVELOPMENT_ENCRYPTION_KEY),
+            );
+        let encrypted_api_key = credential_state
+            .seal_provider_catalog_key_api_key(
+                row.provider_id.as_str(),
+                row.key_id.as_str(),
+                "plain-upstream-key",
+            )
+            .expect("api key should encrypt");
         let key = StoredProviderCatalogKey::new(
             row.key_id.clone(),
             row.provider_id.clone(),
@@ -2185,7 +2199,7 @@ mod tests {
         .expect("key should build")
         .with_transport_fields(
             Some(serde_json::json!([row.endpoint_api_format.clone()])),
-            "plain-upstream-key".to_string(),
+            encrypted_api_key,
             None,
             None,
             None,
@@ -2540,7 +2554,7 @@ mod tests {
                 provider_repository,
                 candidate_repository,
             )
-            .with_encryption_key_for_tests("development-key");
+            .with_encryption_key_for_tests(DEVELOPMENT_ENCRYPTION_KEY);
         let app = AppState::new()
             .expect("gateway state should build")
             .with_data_state_for_tests(data_state);
@@ -2660,7 +2674,7 @@ mod tests {
                 provider_repository,
                 candidate_repository,
             )
-            .with_encryption_key_for_tests("development-key")
+            .with_encryption_key_for_tests(DEVELOPMENT_ENCRYPTION_KEY)
             // Legacy keys deliberately disagree with the routing policy: the
             // resolved policy must be the only source of scheduler ordering.
             .with_system_config_values_for_tests([