Single sign-on servers need no setup step

With single sign-on there is no first-start code and no /setup form: the
identity provider says who may sign in and who is admin, and the first
allowed sign-in makes the organisation, named after ARCHIFY_SYNC_NAME.
Server 0.1.1, chart 0.1.2.

crates/store/src/lib.rs

@@ -313,6 +313,34 @@ impl Store {
         Ok(())
     }
 
+    /// The server's organisation, made with `name` when there is none yet
+    /// (a single sign-on server has no setup step).
+    pub async fn organisation_or_create(&self, name: &str) -> Result<Uuid> {
+        let mut tx = self.pool.begin().await?;
+        // Serialises the first sign-ins, so only one organisation is made.
+        sqlx::query("LOCK TABLE organisations IN EXCLUSIVE MODE")
+            .execute(&mut *tx)
+            .await?;
+        let found: Option<Uuid> =
+            sqlx::query_scalar("SELECT id FROM organisations ORDER BY created_at LIMIT 1")
+                .fetch_optional(&mut *tx)
+                .await?;
+        let id = match found {
+            Some(id) => id,
+            None => {
+                let id = Uuid::now_v7();
+                sqlx::query("INSERT INTO organisations (id, name) VALUES ($1, $2)")
+                    .bind(id)
+                    .bind(name)
+                    .execute(&mut *tx)
+                    .await?;
+                id
+            }
+        };
+        tx.commit().await?;
+        Ok(id)
+    }
+
     /// The server's organisation: the one made at setup.
     pub async fn first_organisation(&self) -> Result<Option<Uuid>> {
         Ok(