feat(admin-console): Stage 1 — pipe + skeleton behind the gateway

Add the server-rendered operator console at /_gm, exposed publicly through
the gateway behind the existing admin_accounts Basic Auth.

Backend:
- new internal/adminconsole package (html/template Renderer, stateless HMAC
  CSRF signer, embedded stylesheet)
- /_gm route group reusing basicauth.Middleware(admin.Service) + a CSRF guard
  (per-operator token + same-origin check); dashboard landing page
- BACKEND_ADMIN_CONSOLE_CSRF_KEY config (per-process random fallback)

Gateway:
- new "admin" public route class (per-IP rate limit, body + GET/HEAD/POST
  method limits) classifying /_gm traffic
- reverse proxy to the backend /_gm surface, preserving Host and relaying the
  backend 401 Basic Auth challenge; 502 when the backend is unreachable
- GATEWAY_PUBLIC_HTTP_ANTI_ABUSE_ADMIN_* config

dev-deploy:
- Caddy routes /_gm/* to the gateway
- bootstrap admin + stable CSRF key; enable Prometheus /metrics exporters on
  backend and gateway (forward-compat for a future Prometheus/Grafana stack)

Docs: ARCHITECTURE 14.1/16, FUNCTIONAL 10.2.1 (+ru mirror), backend and
gateway READMEs, new backend/docs/admin-console.md.

Tests: renderer + CSRF unit tests; backend router auth/render/asset/CSRF;
gateway classifier, proxy forwarding/Host/401/405/413/429/502.

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

@@ -0,0 +1,22 @@
+{{define "content" -}}
+<h1>Dashboard</h1>
+<p class="lede">Signed in as <strong>{{.Username}}</strong>.</p>
+<section class="cards">
+  <a class="card" href="/_gm/users">
+    <h2>Users</h2>
+    <p>Accounts, sanctions, entitlements, soft-delete.</p>
+  </a>
+  <a class="card" href="/_gm/games">
+    <h2>Games &amp; runtimes</h2>
+    <p>Lobby state, engine versions, turn control.</p>
+  </a>
+  <a class="card" href="/_gm/operators">
+    <h2>Operators</h2>
+    <p>Admin accounts: create, disable, reset password.</p>
+  </a>
+  <a class="card" href="/_gm/mail">
+    <h2>Mail &amp; notifications</h2>
+    <p>Deliveries, dead-letters, broadcasts.</p>
+  </a>
+</section>
+{{- end}}