Change a stored app login's metadata.

Any create field; null clears an optional one. A change that ADDS an origin (a new origin or extra origin) deletes every saved secret, so a stored login never follows the app to another site; a new loginMethod or headerKind deletes the secrets the old one used. Each removal is audited. A secret field is refused by name. Free. A story project is refused with 409 project_not_product (nothing is stored or started): app connections are for product projects. Listing, reading and deleting a story project's existing connections still work.