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.

docs/FUNCTIONAL.md

@@ -1162,6 +1162,22 @@ operator's password manager can match it across deployments.
 After the first deployment, the bootstrap password should be
 rotated through the admin surface.
 
+### 10.2.1 Operator console (`/_gm`)
+
+Administrators drive these operations either programmatically through
+the JSON admin API or through a server-rendered web console at `/_gm`.
+The console authenticates with the same Basic Auth credentials: opening
+any `/_gm` page prompts the browser's native credential dialog, and the
+operator stays signed in for the session. Navigation is by ordinary
+links and query parameters; every change is submitted as a form and
+answered with a redirect back to the affected page.
+
+The console is the only admin surface reachable from outside the trusted
+network. It is fronted by the gateway, so it inherits the same edge rate
+limiting and request limits as the public API, and it carries an
+anti-CSRF token on every change. The JSON admin API stays internal to
+the deployment.
+
 ### 10.3 Admin account management
 
 Existing admins can list other admins, create new ones, look up a