Admin name at init, token rotation, read-only forms, token role presets

- stewardd init asks for the first administrator's name on a terminal.
- Tokens can be rotated (steward token rotate, POST /tokens/{id}/rotate,
  Rotate in the web UI), with a grace period for the old secret, as in
  Jarl. Schema migration 2 adds the token columns.
- Read-only users see the settings and a subnet's settings locked,
  instead of editable fields without a Save button.
- The New token dialog only offers roles within what the user may do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/install.md

@@ -57,6 +57,10 @@ sudo chgrp steward /etc/steward /etc/steward/steward.yaml && sudo systemctl daem
    ```sh
    sudo -u steward stewardd init
    ```
+
+   It asks for the administrator's name (`admin` if you just press Enter;
+   your own name is better, since each person should have an account) and
+   password. `-u NAME` gives the name without asking.
 3. **Stop any other DHCP server** on the interfaces Steward will serve
    (Kea, isc-dhcp-server, dnsmasq): only one program can use an
    interface's port 67. Moving from Kea: [kea.md](kea.md).