feat(vk): embed the game as a VK Mini App

Mirror the Telegram Mini App wrapper for VK: the SPA loads at a new /vk/
entry, authenticates from VK's signed launch parameters, and provisions a
'vk' platform identity — the minimum to run the game in VK test mode.

- Gateway verifies the launch signature in-process (internal/vkauth:
  HMAC-SHA256 over the sorted vk_* params under GATEWAY_VK_APP_SECRET,
  base64url) — a pure offline check, no side-service. New auth.vk op
  (gated on the secret), backendclient.VKAuth, /vk/ SPA mount.
- Backend: KindVK + ProvisionVK/vkSeed, /sessions/vk handler, identity
  kind widened to include 'vk' (migration 00005, expand-contract).
- UI: src/lib/vk.ts (VK Bridge, lazy-imported), bootVK + the /vk/ boot
  dispatch, encodeVKLogin + authVK across transport/client/mock. VK omits
  the name from the signed params, so the client reads it via
  VKWebAppGetUserInfo as an unsigned display seed.
- Deploy: /vk in the edge Caddyfile, GATEWAY_VK_APP_SECRET wired through
  compose + .env.example + CI (TEST_) + prod-deploy (PROD_).
- Admin console: surface the VK user id (link to the VK profile) next to
  the Telegram id on the user card.
- Docs: ARCHITECTURE §12/§13, FUNCTIONAL (+ _ru), gateway README; VK
  integration reference under .claude/.

Signature algorithm verified against dev.vk.com plus independent Node/Python
references and a %2C edge-case vector.

ui/src/lib/codec.test.ts

@@ -30,6 +30,7 @@ import {
   encodeTarget,
   encodeTelegramLogin,
   encodeUpdateProfile,
+  encodeVKLogin,
 } from './codec';
 
 describe('codec', () => {
@@ -94,6 +95,13 @@ describe('codec', () => {
     );
     expect(email.email()).toBe('a@example.com');
     expect(email.browserTz()).toBe('+00:00');
+
+    const vk = fb.VKLoginRequest.getRootAsVKLoginRequest(
+      new ByteBuffer(encodeVKLogin('vk_user_id=494075&vk_ts=1&sign=abc', '+03:00', 'Иван Петров')),
+    );
+    expect(vk.params()).toBe('vk_user_id=494075&vk_ts=1&sign=abc');
+    expect(vk.browserTz()).toBe('+03:00');
+    expect(vk.displayName()).toBe('Иван Петров');
   });
 
   it('round-trips a feedback submit and decodes state + unread', () => {