admin dashboard, metrics and telemetry

- embedded React admin UI at /admin/ui (go:embed), login with ADMIN_SECRET
  via signed HttpOnly session cookie, X-ADMIN-KEY still works for scripts
- admin API: overview stats, indexing queue, addon management (priority,
  reindex, delete/restore/purge, bulk), whitelist CRUD, duplicate hashes,
  traffic, metrics explorer, services, events/audit log, system info
- built-in metrics: request latency per route, visitors, top addons and
  searches, indexer/scraper counters; minute buckets in the DB with hourly
  rollups and retention; optional Prometheus /metrics (METRICS_TOKEN)
- POST /back/telemetry for indexers/scrapers (metrics, events, heartbeat)
  and a stdlib-only Go client in telemetry/
- auth hardening: empty secrets no longer allow access, constant-time
  compare, CORS not applied to /admin and /back, TRUSTED_PROXIES setting
- Dockerfile builds the UI and no longer bakes config.yaml into the image;
  CI builds the UI, runs tests and builds arm64 with CGO

web/spa_test.go

@@ -0,0 +1,51 @@
+package web
+
+import (
+	"io/fs"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestSPA(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	Register(r)
+
+	get := func(path string) *httptest.ResponseRecorder {
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, httptest.NewRequest(http.MethodGet, path, nil))
+		return w
+	}
+
+	for _, p := range []string{"/admin/ui/", "/admin/ui/addons/123", "/admin/ui/index.html"} {
+		w := get(p)
+		if w.Code != http.StatusOK || !strings.Contains(w.Header().Get("Content-Type"), "text/html") {
+			t.Errorf("%s: got %d %s", p, w.Code, w.Header().Get("Content-Type"))
+		}
+		if w.Header().Get("Content-Security-Policy") == "" {
+			t.Errorf("%s: missing CSP", p)
+		}
+	}
+
+	if w := get("/admin/ui/assets/missing.js"); w.Code != http.StatusNotFound {
+		t.Errorf("missing asset: got %d, want 404", w.Code)
+	}
+	if w := get("/admin"); w.Code != http.StatusFound || w.Header().Get("Location") != "/admin/ui/" {
+		t.Errorf("/admin: got %d -> %s", w.Code, w.Header().Get("Location"))
+	}
+
+	// when the UI is built, hashed assets are served with long caching
+	sub, _ := fs.Sub(dist, "dist")
+	entries, _ := fs.ReadDir(sub, "assets")
+	for _, e := range entries {
+		w := get("/admin/ui/assets/" + e.Name())
+		if w.Code != http.StatusOK || !strings.Contains(w.Header().Get("Cache-Control"), "immutable") {
+			t.Errorf("asset %s: got %d cache %q", e.Name(), w.Code, w.Header().Get("Cache-Control"))
+		}
+		break
+	}
+}