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

backend/app/settings.py

@@ -31,6 +31,11 @@ class Settings(BaseSettings):
     # Base URL flows use for callbacks into this API. Must be reachable
     # FROM the LFX process (compose: http://api:8000).
     callback_base_url: str = "http://localhost:8000"
+    # Shared secret that authenticates flow callbacks (POST /api/approvals).
+    # The runner injects it into every run envelope; LFX-served flows send
+    # it back as X-Callback-Token. Empty disables callbacks (503) — set a
+    # long random value in prod (.env CALLBACK_TOKEN).
+    callback_token: str = ""
 
     # --- access management (magic-link auth, see app/auth) ---
     # Public frontend URL (magic links point at /login?token= there).