When connecting a Gmail or Outlook inbox during onboarding, the OAuth
flow used to drop users in inbox settings, breaking onboarding. The
OAuth start endpoint now accepts an optional `return_to=onboarding`
hint, carried tamper-proof inside the signed `state`, and the callback
uses it to return the user to the onboarding inbox-setup screen. Without
the hint, behavior is unchanged.
This is the backend half only; the frontend that sends
`return_to=onboarding` ships separately.
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
