fix: authorize flow management, gate runs, token-gate callbacks, strict tenants

.env.example

@@ -24,6 +24,9 @@ LFX_API_KEY=lfx-
 
 # --- API self-reference for flow callbacks (must be reachable FROM lfx) ---
 CALLBACK_BASE_URL=http://api:8000
+# Shared secret for flow callbacks (POST /api/approvals carries it as
+# X-Callback-Token). Long random value; empty disables callbacks.
+CALLBACK_TOKEN=
 
 # --- frontend mode: dev server with HMR (prod .env must set `preview`) ---
 FRONTEND_MODE=dev