Manage master keys through the external API

API keys now see a set of servers instead of one, and /api/v1 gains the
master-key operations the panel already had.

- api_key_servers holds each key's servers (migrated from server_id,
  which stays the default server for /api/v1/users). /users follows
  X-Server-Id within the set; a server outside it is 404. PATCH
  /ui/apikeys/:id changes the set without reissuing the key.
- /api/v1/masterkeys: create (returns the sen:// link), list, get with
  link and devices, patch label/limit/servers, rotate, rekey, delete, and
  nested device routes (unlink, rekey, PSK). A key sees a master key only
  if all of its servers are in the key's set, so it cannot delete or
  re-point service on a server it cannot see; anything else is 404,
  servers outside the set are 403. Creating refuses with 503 before
  writing anything if the subscription cannot issue links.
- sub.ts: the panel routes and the API share one service layer, the
  panel's behaviour is unchanged.
- Node deletion: owners check in beforeDelete (no writes, a later hook
  may still veto) and clean up in afterDelete. Previously a forced delete
  could release a waiting master key and then be vetoed. A node that is
  the only server of an API key is refused; other keys just lose it.
- Panel: server checkboxes are a shared ServerPicker, used by the master
  key settings and the new API key settings row; the API keys table shows
  each key's servers.
- README: the API section covers server sets and master keys.

awg-ui/src/components/ServerPicker.tsx

@@ -0,0 +1,41 @@
+// Copyright (c) 2026 Ivan Vasilev
+// This source code is licensed under the MIT license found in the
+// LICENSE file in the root directory of this source tree.
+import { type NodeInfo, genLabel } from '../lib/shared';
+
+// Выбор набора серверов галочками: серверы мастер-ключа, видимость API-ключа. Набор не
+// бывает пустым — последний отмеченный сервер снять нельзя. onChange получает новый набор
+// и сервер, который переключили.
+export default function ServerPicker({ nodes, selected, onChange }: {
+    nodes: NodeInfo[];
+    selected: number[];
+    onChange: (next: number[], node: NodeInfo, added: boolean) => void;
+}) {
+    return (
+        <>
+            {nodes.map(n => {
+                const on = selected.includes(n.id);
+                const last = on && selected.length === 1;
+                const h = n.health;
+                return (
+                    <label className={`server-pick${last ? ' server-pick--locked' : ''}`} key={n.id}>
+                        <input type="checkbox" checked={on} disabled={last}
+                            onChange={() => onChange(on ? selected.filter(id => id !== n.id) : [...selected, n.id], n, !on)} />
+                        <span className={`chip chip--${n.online ? 'online' : 'offline'} server-pick-dot`} title={n.online ? 'В сети' : 'Нет связи'}>
+                            <span className="chip-dot" />
+                        </span>
+                        <span className="server-pick-name">{h?.server || n.name}</span>
+                        {h?.ip && <code className="server-pick-ip">{h.ip}</code>}
+                        {h?.gen && <span className="server-pick-gen">{genLabel(h.gen)}</span>}
+                    </label>
+                );
+            })}
+        </>
+    );
+}
+
+// Имя сервера для подписей: как в сервер-баре, иначе «#id».
+export const serverName = (nodes: NodeInfo[], id: number): string => {
+    const n = nodes.find(x => x.id === id);
+    return n ? n.health?.server || n.name : `#${id}`;
+};