feat(pool): pin a user to one Claude/Codex pool account

An admin pins a server-mode user to one pool account per tool
(account_pins; PUT /api/accounts/pins with the password step-up and audit,
the Users page's Pin a user, agenthub accounts pin|unpin). The workspace
owner's pin wins over the balancer whenever the pane may use the account;
a pinned login still waits while another pool login holds the home.

server/lib/auth.js

@@ -367,6 +367,7 @@ function deleteUser(username) {
   } catch {
     /* no personal profiles */
   }
+  sql("DELETE FROM account_pins WHERE user_id = ?").run(u.id);
   sql("DELETE FROM users WHERE id = ?").run(u.id);
   return { id: u.id, username: u.username, workspaces: lanes.length };
 }