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

auth/auth_test.go

@@ -0,0 +1,80 @@
+package auth
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.tbdevent.eu/TBD/reforger_crawler_main/initializers"
+)
+
+func TestSecretEqual(t *testing.T) {
+	if SecretEqual("", "") {
+		t.Fatal("empty configured secret must never match")
+	}
+	if SecretEqual("anything", "") {
+		t.Fatal("empty configured secret must never match")
+	}
+	if !SecretEqual("abc", "abc") {
+		t.Fatal("equal secrets must match")
+	}
+	if SecretEqual("abd", "abc") || SecretEqual("ab", "abc") {
+		t.Fatal("different secrets must not match")
+	}
+}
+
+func TestSessionToken(t *testing.T) {
+	initializers.AdminSecret = "first-secret-value"
+	initializers.AdminSessionTTL = time.Hour
+	now := time.Now()
+
+	tok, exp := IssueToken(now)
+	got, ok := VerifyToken(tok, now)
+	if !ok || got.Unix() != exp.Unix() {
+		t.Fatalf("fresh token should verify, ok=%v", ok)
+	}
+
+	if _, ok := VerifyToken(tok, now.Add(2*time.Hour)); ok {
+		t.Fatal("expired token must not verify")
+	}
+
+	parts := strings.Split(tok, ".")
+	parts[1] = "9999999999"
+	if _, ok := VerifyToken(strings.Join(parts, "."), now); ok {
+		t.Fatal("tampered token must not verify")
+	}
+
+	if _, ok := VerifyToken("garbage", now); ok {
+		t.Fatal("garbage must not verify")
+	}
+
+	initializers.AdminSecret = "rotated-secret-value"
+	if _, ok := VerifyToken(tok, now); ok {
+		t.Fatal("rotating the secret must invalidate sessions")
+	}
+
+	initializers.AdminSecret = ""
+	if _, ok := VerifyToken(tok, now); ok {
+		t.Fatal("no secret configured must reject all sessions")
+	}
+}
+
+func TestLoginLimiter(t *testing.T) {
+	l := NewLoginLimiter(3, time.Minute)
+	for range 3 {
+		if l.Blocked("1.2.3.4") {
+			t.Fatal("blocked too early")
+		}
+		l.Fail("1.2.3.4")
+	}
+	if !l.Blocked("1.2.3.4") {
+		t.Fatal("should be blocked after max failures")
+	}
+	if l.Blocked("5.6.7.8") {
+		t.Fatal("other IPs must not be affected")
+	}
+	l.Reset("1.2.3.4")
+	if l.Blocked("1.2.3.4") {
+		t.Fatal("reset should unblock")
+	}
+}