MCP sign-in redirect hangs indefinitely at /sign-in?redirect_url=/oauth/consent/...

## Bug description
When clicking "Sign in" on an MCP app page (e.g. https://mcp.pipedream.com/app/google_sheets) to connect an account, the browser navigates to `https://mcp.pipedream.com/sign-in?redirect_url=/oauth/consent/{id}` and the page gets stuck indefinitely on "Redirecting to sign in..." — it never proceeds to an actual sign-in form or OAuth consent screen.

## Steps to reproduce
1. Go to https://mcp.pipedream.com/app/google_sheets
2. Click "Sign in"
3. Page navigates to `/sign-in?redirect_url=/oauth/consent/`
4. Page shows "Redirecting to sign in..." and never proceeds

## Expected behavior
Should redirect to the actual sign-in / OAuth consent flow.

## Additional context
Reproduced independently on two different devices/networks/browsers, ruling out local browser/extension/network issues. Earlier the same day, `/api/app/google_sheets` and `/api/actions/google_sheets` returned HTTP 500 (later resolved), suggesting a related backend deploy issue.