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/render.go

@@ -0,0 +1,107 @@
+package adminconsole
+
+import (
+	"bytes"
+	"embed"
+	"fmt"
+	"html/template"
+	"io"
+	"io/fs"
+	"path"
+	"strings"
+)
+
+//go:embed templates
+var templatesFS embed.FS
+
+//go:embed assets
+var assetsFS embed.FS
+
+// Renderer holds the parsed admin console templates. It composes one template
+// set per content page, each combining the shared layout (defining the page
+// chrome and the "layout" entry template) with that page's "content" block, so
+// rendering a page is a single ExecuteTemplate call against the "layout" name.
+type Renderer struct {
+	pages map[string]*template.Template
+}
+
+// PageData is the view model passed to every admin console page. Title is the
+// document title; Username is the authenticated operator; CSRFToken is the
+// per-operator token embedded into state-changing forms; ActiveNav marks the
+// highlighted navigation entry; Data carries the page-specific payload.
+type PageData struct {
+	Title     string
+	Username  string
+	CSRFToken string
+	ActiveNav string
+	Data      any
+}
+
+// NewRenderer parses the embedded layout and every content page under
+// templates/pages, returning a Renderer ready to serve them. It fails when a
+// template cannot be parsed.
+func NewRenderer() (*Renderer, error) {
+	base, err := template.New("layout").ParseFS(templatesFS, "templates/layout.gohtml")
+	if err != nil {
+		return nil, fmt.Errorf("parse admin console layout: %w", err)
+	}
+
+	pageFiles, err := fs.Glob(templatesFS, "templates/pages/*.gohtml")
+	if err != nil {
+		return nil, fmt.Errorf("enumerate admin console pages: %w", err)
+	}
+	if len(pageFiles) == 0 {
+		return nil, fmt.Errorf("admin console: no page templates found under templates/pages")
+	}
+
+	pages := make(map[string]*template.Template, len(pageFiles))
+	for _, file := range pageFiles {
+		name := strings.TrimSuffix(path.Base(file), ".gohtml")
+		clone, err := base.Clone()
+		if err != nil {
+			return nil, fmt.Errorf("clone admin console layout for %q: %w", name, err)
+		}
+		if _, err := clone.ParseFS(templatesFS, file); err != nil {
+			return nil, fmt.Errorf("parse admin console page %q: %w", name, err)
+		}
+		pages[name] = clone
+	}
+
+	return &Renderer{pages: pages}, nil
+}
+
+// MustNewRenderer is like NewRenderer but panics on error. The templates are
+// embedded at build time, so a parse failure is a programmer error rather than
+// a runtime condition.
+func MustNewRenderer() *Renderer {
+	renderer, err := NewRenderer()
+	if err != nil {
+		panic(err)
+	}
+	return renderer
+}
+
+// Render writes the named page, wrapped in the shared layout, to w using data.
+// It returns an error when page is unknown or template execution fails; the
+// page is rendered into an intermediate buffer first so a mid-render failure
+// never emits a partial document to w.
+func (r *Renderer) Render(w io.Writer, page string, data PageData) error {
+	tmpl, ok := r.pages[page]
+	if !ok {
+		return fmt.Errorf("admin console: unknown page %q", page)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.ExecuteTemplate(&buf, "layout", data); err != nil {
+		return fmt.Errorf("render admin console page %q: %w", page, err)
+	}
+
+	_, err := buf.WriteTo(w)
+	return err
+}
+
+// Assets returns the embedded static asset tree rooted at the assets directory,
+// suitable for serving under `/_gm/assets/`.
+func Assets() (fs.FS, error) {
+	return fs.Sub(assetsFS, "assets")
+}