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.

docker-compose.single-node.yml

@@ -2,7 +2,14 @@ services:
   app:
     image: ${APP_IMAGE:-ghcr.io/fawney19/aether:latest}
     container_name: aether-app
-    user: "0:0"
+    user: "${AETHER_CONTAINER_UID:-65532}:${AETHER_CONTAINER_GID:-65532}"
+    read_only: true
+    cap_drop:
+      - ALL
+    security_opt:
+      - no-new-privileges:true
+    tmpfs:
+      - /tmp:rw,nosuid,nodev,noexec,mode=1777
     env_file:
       - ${AETHER_ENV_FILE:-.env}
     environment: