Single sign-on is the only way in when it is configured

The sign-in page shows only the identity provider; password sign-in and
new local accounts are refused. Invites go through the identity provider.
Server 0.1.2, chart 0.1.3.

README.md

@@ -16,7 +16,7 @@ One Rust process and PostgreSQL. The design, with its decisions, is the Archify
   - **Update log:** taking a version compacts it.
 - **Sign-in.** The editor opens `/authorize` in the browser and gets a one-time code back through its URI handler. It exchanges the code with its PKCE verifier for an access token (JWT, 15 minutes) and a refresh token. The refresh token is single use: reusing one signs out the whole session.
 - **First start.** With single sign-on there is no setup step: the identity provider says who may sign in and who is admin, and the first sign-in makes the organisation (named after `ARCHIFY_SYNC_NAME`). Without single sign-on, a server without accounts prints a first-start code in its log; the first person to use it at `/setup` becomes the admin.
-- **Invites.** An invite link (`/join/<token>`) opens the extension. Someone new makes an account on the sign-in page, so no mail server is needed.
+- **Invites.** An invite link (`/join/<token>`) opens the extension. Someone new makes an account on the sign-in page, so no mail server is needed. With single sign-on, the identity provider is the only way in: the sign-in page has no password form, and an invited person signs in there.
 
 ## Layout