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/README.md

@@ -27,10 +27,16 @@ The implementation specification lives in `PLAN.md`.
 | ------------------ | ----------------------------------------------- | ------------------------------------- |
 | `/api/v1/public/*` | none                                            | Registration, code confirmation       |
 | `/api/v1/user/*`   | `X-User-ID` injected by gateway                 | Authenticated end users               |
-| `/api/v1/admin/*`  | HTTP Basic Auth against `admin_accounts`        | Platform administrators               |
+| `/api/v1/admin/*`  | HTTP Basic Auth against `admin_accounts`        | Platform administrators (JSON)        |
+| `/_gm`, `/_gm/*`   | HTTP Basic Auth against `admin_accounts`        | Operator console (server-rendered HTML)|
 | `/healthz`         | none                                            | Liveness probe                        |
 | `/readyz`          | none                                            | Readiness probe                       |
 
+The `/_gm` operator console is the human-facing surface for the admin
+operations; it reuses the admin Basic Auth verifier, renders with
+`html/template`, and is the only admin surface exposed publicly (through
+the gateway). See `docs/admin-console.md`.
+
 The full contract is documented in `openapi.yaml` and validated at
 runtime by the contract tests under `internal/server/`.
 
@@ -100,6 +106,7 @@ fast.
 | `BACKEND_GAME_STATE_ROOT`               | yes      | —                             | Host directory bind-mounted into engine containers.   |
 | `BACKEND_ADMIN_BOOTSTRAP_USER`          | no       | —                             | Initial admin username; idempotent insert.            |
 | `BACKEND_ADMIN_BOOTSTRAP_PASSWORD`      | no       | —                             | Initial admin password; required if user is set.      |
+| `BACKEND_ADMIN_CONSOLE_CSRF_KEY`        | no       | random per-process            | Secret keying the `/_gm` console CSRF token. Set a shared value across replicas; unset uses a per-process random key (forms reset on restart). |
 | `BACKEND_GEOIP_DB_PATH`                 | yes      | —                             | Filesystem path to GeoLite2 Country `.mmdb`.          |
 | `BACKEND_OTEL_TRACES_EXPORTER`          | no       | `otlp`                        | `none`, `otlp`, `stdout`.                             |
 | `BACKEND_OTEL_METRICS_EXPORTER`         | no       | `otlp`                        | `none`, `otlp`, `stdout`, `prometheus`.               |