Start a bank connection (get the consent link)

Starts a bank connection for the authenticated user and returns the authorizationUri you must open in the user's browser so they can log into their bank and grant consent.

Flow: (1) call this with the providerId chosen from GET /bank/providers; (2) redirect/open the returned authorizationUri — the user authenticates and consents at their own bank (we never see their bank credentials); (3) after they return, call POST /bank/fetch, which confirms the consent and returns their transactions.

A pending connection record is stored immediately; it becomes active on the first successful fetch. providerId is required and must be chosen from the available bank directory. Only account-information read access is supported.