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/tabs/apikeys/apikeys.css

@@ -75,3 +75,18 @@ LICENSE file in the root directory of this source tree.
 @media (max-width: 640px) {
     .apikey-cards-list { display: flex; }
 }
+
+/* Настройки ключа: какие серверы он видит */
+.apikey-tr-tools td { padding-top: 0; }
+.apikey-tools {
+    display: flex; flex-direction: column; gap: 10px;
+    background: var(--surface-bright);
+    border: 1px solid var(--outline-variant);
+    border-radius: 16px;
+    padding: 12px 16px 14px;
+    margin: 6px 0 8px;
+}
+.apikey-tools-title { margin: 0; font-size: 13px; font-weight: 500; color: var(--label); }
+.apikey-tools-hint { margin: 0; font-size: 12px; color: var(--neutral-text); }
+.apikey-card-btns { display: flex; gap: 8px; flex-shrink: 0; }
+.apikey-card-servers { font-size: 12px; color: var(--neutral-text); }