chore: initialize standalone business plugin repository

plugins/subscription-admin/.env.example

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+CORE_BASE_URL=http://127.0.0.1:8080
+PLUGIN_HOST=127.0.0.1
+PLUGIN_PORT=8091
+# Optional public path when mounted behind a reverse proxy, e.g.
+# /extensions/qiu.subscription-admin
+PLUGIN_PUBLIC_BASE_PATH=
+# Limit the session cookie to the plugin mount path in production.
+PLUGIN_COOKIE_PATH=
+# Set true only behind HTTPS. Non-loopback listeners fail closed when false.
+PLUGIN_COOKIE_SECURE=false
+# lax (same-site proxy), strict, or none (cross-site iframe; requires Secure).
+PLUGIN_COOKIE_SAMESITE=lax
+# Space-separated frame ancestors. Keep 'self' for same-origin proxying.
+PLUGIN_FRAME_ANCESTORS='self'
+# Set true only when the immediate reverse proxy is trusted and supplies XFF.
+PLUGIN_TRUST_PROXY=false