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

models/addon.go

@@ -27,6 +27,7 @@ type Addon struct {
 	PriorityIndexing     bool           `json:"priorityIndexing" gorm:"default:false"`
 	AddonFiles           []AddonFile    `json:"addonFiles"`
 	Recheck              bool           `json:"recheck" gorm:"default:false"`
+	LastIndexedAt        *time.Time     `json:"lastIndexedAt"`
 
 	Preview string `json:"preview"`
 	Author  string `json:"author"`