Disable reservations; subnet enable/disable buttons; page-head CSS fix

- A disabled reservation is kept but ignored: the client gets a pool
  address like any other (steward reservation disable|enable, buttons and
  a checkbox in the web UI). Schema migration 3.
- Subnets get Disable/Enable buttons on their page and in the list.
- A rule left without its selector in style.css broke the page heading's
  layout, so the New ... buttons sat cramped under the description.

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

docs/cli.md

@@ -79,6 +79,7 @@ one. An option Steward doesn't know: `--option 224:string=hello`.
 | `reservation show RES` | RES is its ID, address or MAC address |
 | `reservation add IP MAC [--hostname H] [--client-id HEX] [--option NAME=VALUE]... [--comment C]` | `-` for MAC with `--client-id` |
 | `reservation update RES [--ip] [--mac] [--client-id] [--hostname] [--option]... [--unset-option]... [--comment]` | |
+| `reservation enable\|disable RES` | use a reservation, or keep it but ignore it |
 | `reservation delete RES` | |
 
 ### lease