Pull the HTTP-independent core out of the v1 auth handlers so both
/api/v1 and the upcoming /api/v2 routes share one implementation:
- oauth2server: ExchangeToken and Authorize take plain inputs and return
typed responses; HandleToken/HandleAuthorize keep binding + headers.
- pkg/routes/api/shared: AuthenticateLinkShare, RegisterUser,
ResetPassword (+ session clear), RequestPasswordResetToken and
ConfirmEmail, plus the shared UserRegister and LinkShareToken types.
v1 handlers now delegate to these; their wire output is unchanged.
kolaente