Give master keys a stable uuid

- master_keys.uuid (UUID v4, unique index): new keys get one on create,
  existing keys once on start. It survives rotate (new secret) and
  relabel, so it identifies the key itself rather than its link.
- /ui/masterkeys* and /api/v1/masterkeys* return uuid, and :id in those
  paths accepts the numeric id or the uuid.
- /sub/v1 register, peek and devices return key_uuid, so a client can
  recognise the same key behind a reissued link. It is not in the sen://
  link (that would need a new format) and not in config, so rev and
  existing clients are unaffected - checked against a 0.3.1 ui.db.
- Panel: the uuid sits in the key's row like a device ID (full value in
  the tooltip, click copies) and in the key settings with a copy button;
  on mobile it is shown under the key's name.
- README: uuid in the master key API.

awg-ui/mock-server.ts

@@ -50,12 +50,12 @@ let apiId = 1;
 
 // ── мастер-ключи sen:// ──
 interface Dev { id: number; device_id: string; device_name: string; platform: string; version: string; created_at: number; last_seen: number | null; rekey_requested: boolean; online: boolean; lastHandshake: number; rx: number; tx: number }
-interface MK { id: number; label: string; device_limit: number; created_at: number; secret: Buffer; devs: Dev[]; servers: number[]; deleting?: number[] }
+interface MK { id: number; uuid: string; label: string; device_limit: number; created_at: number; secret: Buffer; devs: Dev[]; servers: number[]; deleting?: number[] }
 let mkId = 1, devId = 1;
 const dev = (name: string, platform: string, online: boolean, version = "0.6.5"): Dev => ({ id: devId++, device_id: crypto.randomUUID(), device_name: name, platform, version, created_at: now() - 86400, last_seen: now() - rnd(900), rekey_requested: false, online, lastHandshake: now() - rnd(300), rx: rnd(9e8), tx: rnd(3e8) });
-const masters: MK[] = [{ id: mkId++, label: "Семья", device_limit: 3, created_at: now() - 86400 * 2, secret: crypto.randomBytes(16), devs: [dev("MacBook Ивана", "macOS", true), dev("Windows-ПК", "Windows", false, "0.6.2")], servers: [0, 1] }];
+const masters: MK[] = [{ id: mkId++, uuid: crypto.randomUUID(), label: "Семья", device_limit: 3, created_at: now() - 86400 * 2, secret: crypto.randomBytes(16), devs: [dev("MacBook Ивана", "macOS", true), dev("Windows-ПК", "Windows", false, "0.6.2")], servers: [0, 1] }];
 const signPub = crypto.randomBytes(32);
-const mkOut = (m: MK) => ({ id: m.id, label: m.label, device_limit: m.device_limit, devices: m.devs.length, servers: m.servers, created_at: m.created_at, ...(m.deleting ? { deleting: m.deleting } : {}) });
+const mkOut = (m: MK) => ({ id: m.id, uuid: m.uuid, label: m.label, device_limit: m.device_limit, devices: m.devs.length, servers: m.servers, created_at: m.created_at, ...(m.deleting ? { deleting: m.deleting } : {}) });
 // Мок: устройство «догнано» на всех серверах ключа, кроме Tokyo (id 2 — ещё не подключалась).
 const devOut = (m: MK, d: Dev) => ({ ...d, servers_total: m.servers.length, servers_ok: m.servers.filter(s => s !== 2).length });
 const find = (id: string) => masters.find(m => m.id === Number(id) && !m.deleting);
@@ -123,7 +123,7 @@ app.get("/ui/masterkeys", (_q, r) => r.json({ enabled: true, tls: false, keys: m
 app.post("/ui/masterkeys", (q, r) => {
     const limit = q.body.device_limit ?? 3;
     if (!q.body.label || !Number.isInteger(limit) || limit < 1 || limit > 100) return r.status(400).json({ error: "Неверные параметры" });
-    const m: MK = { id: mkId++, label: q.body.label, device_limit: limit, created_at: now(), secret: crypto.randomBytes(16), devs: [], servers: q.body.servers ?? [0] };
+    const m: MK = { id: mkId++, uuid: crypto.randomUUID(), label: q.body.label, device_limit: limit, created_at: now(), secret: crypto.randomBytes(16), devs: [], servers: q.body.servers ?? [0] };
     masters.push(m); r.status(201).json(mkOut(m));
 });
 app.patch("/ui/masterkeys/:id", (q, r) => {