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.ts

@@ -189,6 +189,18 @@ export function encodeTelegramLogin(initData: string, browserTz: string): Uint8A
   return finish(b, fb.TelegramLoginRequest.endTelegramLoginRequest(b));
 }
 
+export function encodeVKLogin(params: string, browserTz: string, displayName: string): Uint8Array {
+  const b = new Builder(512);
+  const p = b.createString(params);
+  const tz = b.createString(browserTz);
+  const dn = b.createString(displayName);
+  fb.VKLoginRequest.startVKLoginRequest(b);
+  fb.VKLoginRequest.addParams(b, p);
+  fb.VKLoginRequest.addBrowserTz(b, tz);
+  fb.VKLoginRequest.addDisplayName(b, dn);
+  return finish(b, fb.VKLoginRequest.endVKLoginRequest(b));
+}
+
 export function encodeGuestLogin(locale: string, browserTz: string): Uint8Array {
   const b = new Builder(64);
   const l = b.createString(locale);