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/metrics.go

@@ -0,0 +1,74 @@
+package models
+
+import "time"
+
+// All timestamps in the metrics tables are int64 unix seconds (events: unix
+// milliseconds) so range queries behave the same on SQLite and Postgres.
+
+// MetricSeries identifies one time series: a metric name plus a fixed label set
+// reported by one service instance.
+type MetricSeries struct {
+	ID        uint   `gorm:"primaryKey" json:"id"`
+	Service   string `gorm:"size:64;not null;uniqueIndex:ux_metric_series,priority:1" json:"service"`
+	Instance  string `gorm:"size:128;not null;uniqueIndex:ux_metric_series,priority:2" json:"instance"`
+	Name      string `gorm:"size:96;not null;uniqueIndex:ux_metric_series,priority:3;index" json:"name"`
+	Labels    string `gorm:"size:512;not null;uniqueIndex:ux_metric_series,priority:4" json:"labels"` // canonical JSON object
+	Kind      string `gorm:"size:16;not null" json:"kind"`                                            // counter|gauge|summary|histogram
+	Bounds    string `gorm:"type:text" json:"bounds"`                                                 // JSON []float64, histograms only
+	Help      string `gorm:"size:256" json:"help"`
+	CreatedAt time.Time
+	LastSeen  int64 `gorm:"index" json:"lastSeen"`
+}
+
+// MetricPoint is one aggregated bucket of a series. Resolution is the bucket
+// width in seconds (60 = raw minute data, 3600 = hourly rollup).
+type MetricPoint struct {
+	SeriesID   uint    `gorm:"primaryKey;autoIncrement:false"`
+	Resolution int     `gorm:"primaryKey;autoIncrement:false;index:idx_metric_points_res_bucket,priority:1"`
+	Bucket     int64   `gorm:"primaryKey;autoIncrement:false;index:idx_metric_points_res_bucket,priority:2"`
+	Count      int64   `gorm:"not null;default:0"`
+	Sum        float64 `gorm:"not null;default:0"`
+	Min        float64 `gorm:"not null;default:0"`
+	Max        float64 `gorm:"not null;default:0"`
+	Last       float64 `gorm:"not null;default:0"`
+	Hist       string  `gorm:"type:text"` // JSON []uint64, histograms only
+}
+
+// ServiceInstance is one running process (indexer, scraper, the API itself)
+// that reports telemetry. Every telemetry push counts as a heartbeat.
+type ServiceInstance struct {
+	ID                   uint      `gorm:"primaryKey" json:"id"`
+	Service              string    `gorm:"size:64;not null;uniqueIndex:ux_service_instance,priority:1" json:"service"`
+	Instance             string    `gorm:"size:128;not null;uniqueIndex:ux_service_instance,priority:2" json:"instance"`
+	Version              string    `gorm:"size:64" json:"version"`
+	Status               string    `gorm:"size:16" json:"status"` // self-reported: ok|degraded|error
+	StatusMessage        string    `gorm:"size:512" json:"statusMessage"`
+	StartedAt            int64     `json:"startedAt"`
+	LastSeen             int64     `gorm:"index" json:"lastSeen"`
+	HeartbeatIntervalSec int       `json:"heartbeatIntervalSec"`
+	RemoteAddr           string    `gorm:"size:64" json:"remoteAddr"`
+	Meta                 string    `gorm:"type:text" json:"meta"` // JSON object
+	CreatedAt            time.Time `json:"createdAt"`
+	UpdatedAt            time.Time `json:"updatedAt"`
+}
+
+// ServiceEvent is a log line reported by a service. Admin actions are recorded
+// here too (service "main", fields.kind "audit").
+type ServiceEvent struct {
+	ID       uint   `gorm:"primaryKey" json:"id"`
+	Service  string `gorm:"size:64;index:idx_service_events_svc_ts,priority:1" json:"service"`
+	Instance string `gorm:"size:128" json:"instance"`
+	Level    string `gorm:"size:8;index" json:"level"` // debug|info|warn|error
+	Message  string `gorm:"size:1024" json:"message"`
+	Fields   string `gorm:"type:text" json:"fields"`                                    // JSON object
+	TS       int64  `gorm:"index:idx_service_events_svc_ts,priority:2;index" json:"ts"` // unix ms
+}
+
+// DailyTopItem counts how often something was requested on one UTC day
+// (Kind "addon": addon lookups, Kind "search": search queries).
+type DailyTopItem struct {
+	Day   string `gorm:"primaryKey;size:10"` // YYYY-MM-DD
+	Kind  string `gorm:"primaryKey;size:16"`
+	Item  string `gorm:"primaryKey;size:128"`
+	Count int64
+}