Reauthorise Connection

Reconnects an existing connection in place. On OAuth platforms it starts a fresh sign-in and returns the connection with a new authorisation URL to send the browser to; on API-key platforms pass the fresh key as 'apiKey' and it is validated live onto the same row. Use this for "Reconnect" rather than creating a connection: the row keeps its code, so documents, folder mappings, client books links and bookkeeping health links stay attached, and platforms whose refresh token rotates never end up with two rows holding one grant. Creating a connection remains the route for a genuinely different account.