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.

backend/internal/adminconsole/templates/pages/user_detail.gohtml

@@ -142,6 +142,11 @@
 {{else}}<p class="note">connector not configured (set BACKEND_CONNECTOR_ADDR)</p>{{end}}
 </section>
 {{end}}
+{{if .VKID}}
+<section class="panel"><h2>VK</h2>
+<p>VK ID: <code>{{.VKID}}</code> · <a href="https://vk.com/id{{.VKID}}" target="_blank" rel="noopener">open profile</a></p>
+</section>
+{{end}}
 <section class="panel"><h2>Games</h2>
 <table class="list">
 <thead><tr><th>Game</th><th>Variant</th><th>Status</th><th class="num">Players</th><th>Updated</th></tr></thead>