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.

gateway/cmd/gateway/main.go

@@ -78,6 +78,15 @@ func run(ctx context.Context) (err error) {
 		AuthService: authServiceAdapter{rest: backend.REST()},
 	}
 
+	adminConsoleProxy, err := restapi.NewBackendConsoleProxy(cfg.Backend.HTTPBaseURL, logger)
+	if err != nil {
+		_ = backend.Close()
+		_ = telemetryRuntime.Shutdown(context.Background())
+		_ = logging.Sync(logger)
+		return fmt.Errorf("build admin console proxy: %w", err)
+	}
+	publicRESTDeps.AdminConsoleProxy = adminConsoleProxy
+
 	grpcDeps, components, cleanup, err := newAuthenticatedGRPCDependencies(ctx, cfg, logger, telemetryRuntime, backend)
 	if err != nil {
 		_ = backend.Close()