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/handlers/proxy/websocket/responses/mod.rs

@@ -18,6 +18,7 @@ mod frame;
 mod lifecycle;
 mod observation;
 mod ownership;
+mod plan_admission;
 mod quota;
 mod redaction;
 mod relay_policy;