fix(deploy): unify backend URL policy

docker/smoke/frontend-policy-smoke.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+set -eu
+
+corpus=/etc/thothii/backend-url-cases.json
+
+jq -c '.[]' "$corpus" | while IFS= read -r case_json; do
+  value=$(printf '%s' "$case_json" | jq -r '.value')
+  valid=$(printf '%s' "$case_json" | jq -r '.valid')
+  if BACKEND_BASE_URL="$value" /usr/local/bin/frontend-entrypoint true \
+      >/dev/null 2>&1; then
+    actual=true
+  else
+    actual=false
+  fi
+  if [ "$actual" != "$valid" ]; then
+    echo "entrypoint policy mismatch for BACKEND_BASE_URL=$value: expected $valid" >&2
+    exit 1
+  fi
+done
+
+echo "frontend entrypoint canonical URL corpus: ok"