Invite a client account to a manager

Live manager (MCC) and client tree for a Google Ads connection. Starts from every customer the Google user behind the connection can access directly and walks each tree to any depth with `customer_client`, then reads each manager's own client links so every client carries its direct parent, the `managerLinkId` and the link status. Invitations a manager sent that the client has not accepted yet appear as clients with `linkStatus: PENDING` (Google returns no name or currency for them). Refused, canceled and ended links are history and are omitted. `managerLinks` on a root lists the managers linked to that account, including invitations it can still accept with PATCH /v1/ads/accounts/manager-links. A directly accessible account that is also nested in another tree appears only once, inside that tree. `directCustomers` lists every account the Google user accesses directly (the ones this connection can accept or decline invitations for) with their pending invitations, including accounts that are also nested in a tree. Customers Google refuses to read (for example a cancelled account) are listed in `unavailable` with Google's reason instead of failing the call. Up to 50 roots and 50 managers per root are read; `truncated` is true when more exist. Cached for 10 minutes per connection; the response carries `cachedAt` and `stale`. When the connection is scoped to specific ad accounts, client accounts outside that scope are hidden (managers stay visible).