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.

tests/install_local_bundle_safety_test.sh

@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+REPO_ROOT="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd -P)"
+# shellcheck source=../install.sh
+source "${REPO_ROOT}/install.sh"
+
+TEST_ROOT="$(mktemp -d)"
+trap 'rm -rf -- "${TEST_ROOT}"' EXIT
+
+fail_test() {
+    echo "FAIL: $*" >&2
+    exit 1
+}
+
+make_bundle() {
+    local bundle="$1"
+    mkdir -p "${bundle}/bin" "${bundle}/frontend/assets"
+    printf '#!/bin/sh\nexit 0\n' >"${bundle}/bin/aether-gateway"
+    printf '<!doctype html>\n' >"${bundle}/frontend/index.html"
+    printf 'asset\n' >"${bundle}/frontend/assets/app.js"
+    chmod 0755 "${bundle}/bin/aether-gateway"
+}
+
+assert_rejected() {
+    local bundle="$1"
+    local description="$2"
+    if (validate_local_bundle_tree "${bundle}"); then
+        fail_test "unsafe local bundle was accepted: ${description}"
+    fi
+}
+
+valid="${TEST_ROOT}/valid"
+make_bundle "${valid}"
+validate_local_bundle_tree "${valid}"
+
+victim="${TEST_ROOT}/victim"
+printf 'do-not-touch\n' >"${victim}"
+
+frontend_link="${TEST_ROOT}/frontend-link"
+make_bundle "${frontend_link}"
+rm -rf -- "${frontend_link}/frontend"
+ln -s -- "${TEST_ROOT}" "${frontend_link}/frontend"
+assert_rejected "${frontend_link}" "frontend symbolic link"
+[[ "$(cat "${victim}")" == "do-not-touch" ]] \
+    || fail_test "frontend symbolic-link validation modified its target"
+
+binary_link="${TEST_ROOT}/binary-link"
+make_bundle "${binary_link}"
+rm -f -- "${binary_link}/bin/aether-gateway"
+ln -s -- "${valid}/bin/aether-gateway" "${binary_link}/bin/aether-gateway"
+assert_rejected "${binary_link}" "binary symbolic link"
+
+hardlink="${TEST_ROOT}/hardlink"
+make_bundle "${hardlink}"
+ln "${hardlink}/frontend/index.html" "${hardlink}/frontend/index-copy.html"
+assert_rejected "${hardlink}" "multiply-linked frontend file"
+
+fifo="${TEST_ROOT}/fifo"
+make_bundle "${fifo}"
+mkfifo "${fifo}/frontend/events.pipe"
+assert_rejected "${fifo}" "FIFO frontend entry"
+
+bundle_link="${TEST_ROOT}/bundle-link"
+ln -s -- "${valid}" "${bundle_link}"
+assert_rejected "${bundle_link}" "bundle symbolic link"
+
+echo "PASS: local release bundle type and link safety fixtures"