feat: bootstrap backend MVP and architecture docs

backend/README.md

@@ -0,0 +1,25 @@
+# memberapi.ose.tw backend
+
+## Quick start
+
+```bash
+cd backend
+python -m venv .venv
+source .venv/bin/activate
+pip install -e .
+cp .env.example .env
+uvicorn app.main:app --host 127.0.0.1 --port 8000 --reload
+```
+
+## Required DB setup
+
+1. Initialize API client whitelist table with `docs/API_CLIENTS_SQL.sql`.
+2. Initialize core tables with `backend/scripts/init_schema.sql`.
+
+## Main APIs
+
+- `GET /healthz`
+- `POST /internal/users/upsert-by-sub`
+- `GET /internal/permissions/{authentik_sub}/snapshot`
+- `POST /admin/permissions/grant`
+- `POST /admin/permissions/revoke`