fix: guard against duplicate callback — skip token exchange if already stored

src/routes/demo.py

@@ -402,6 +402,11 @@ async def tink_callback(request: Request, code: Optional[str] = None,
         print(f"[CALLBACK] Tink returned error: {error}")
         return RedirectResponse(f"/demo/step/3?error={error}")
     if code:
+        # Guard: if we already have a user_token for this session, the code was
+        # already exchanged (duplicate callback from Traefik during rolling deploy).
+        if _load_token(sess, "user_token"):
+            print(f"[CALLBACK] Already have user_token — skipping duplicate exchange")
+            return RedirectResponse("/demo/step/3?cb_success=1", status_code=303)
         try:
             s = get_settings()
             print(f"[CALLBACK] Exchanging code, redirect_uri={s.tink_redirect_uri!r}")