restructure: move backend into backend/

backend/.env.example

@@ -0,0 +1,17 @@
+# --- Database (FreeRADIUS) ---
+DB_HOST=192.168.1.21
+DB_PORT=3306
+DB_USER=root
+DB_PASSWORD=changeme
+DB_NAME=radius
+
+# --- API ---
+# Shared secret required in the X-API-Key header on every request.
+API_KEY=change-this-to-a-long-random-string
+
+# Comma-separated CORS origins (use * only for trusted/dev networks)
+CORS_ORIGINS=*
+
+# Default page size cap for list endpoints
+DEFAULT_LIMIT=50
+MAX_LIMIT=500