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/csrf_test.go

@@ -0,0 +1,42 @@
+package adminconsole
+
+import "testing"
+
+func TestCSRFTokenRoundTrip(t *testing.T) {
+	signer := NewCSRF([]byte("shared-secret"))
+	token := signer.Token("alice")
+
+	if !signer.Verify("alice", token) {
+		t.Fatal("valid token rejected")
+	}
+	if signer.Verify("bob", token) {
+		t.Fatal("token accepted for a different operator")
+	}
+	if signer.Verify("alice", "") {
+		t.Fatal("empty token accepted")
+	}
+	if signer.Verify("alice", token+"x") {
+		t.Fatal("tampered token accepted")
+	}
+}
+
+func TestCSRFKeySeparation(t *testing.T) {
+	a := NewCSRF([]byte("key-a"))
+	b := NewCSRF([]byte("key-b"))
+	if a.Token("operator") == b.Token("operator") {
+		t.Fatal("tokens collide across distinct keys")
+	}
+	if b.Verify("operator", a.Token("operator")) {
+		t.Fatal("token minted under one key verified under another")
+	}
+}
+
+func TestRandomCSRFRoundTrip(t *testing.T) {
+	signer, err := NewRandomCSRF()
+	if err != nil {
+		t.Fatalf("NewRandomCSRF: %v", err)
+	}
+	if !signer.Verify("operator", signer.Token("operator")) {
+		t.Fatal("random-key token failed to round-trip")
+	}
+}