hub v0.64.0 — offsite pool-box aggregate: fill, oversubscription, per-customer bars, operator alert (R-5)

The operator sees the shared pool box's real state on the hub: total box fill vs
capacity, Σ(shared soft quotas) vs capacity (the oversubscription ratio), per-customer
usage/quota bars, and a box-level operator alert (fill % + oversub ratio) on the existing
dispatcher's operator channel. Per-customer fill alerts already existed; the box-level
aggregate was the gap. READ-ONLY against Hetzner (GET only).

Phase-0 probe (gate PASSED): the live pool box 611714 returns capacity via
storage_box_type.size (1 TiB / bx11) and usage via a stats object (size/size_data/
size_snapshots), all bytes; our token reads it (200).

- hetznerapi: additive StorageBoxType + StorageBoxStats on StorageBox (no existing field/
  method changed); fake carries them + a GetBoxCalls counter; golden decode test.
- monitor.OffsiteBoxChecker: OffsiteChecker-sibling for the box; fetch-throttled (1 GET/
  15min), cached BoxSnapshot, escalation-only + recovery re-arm. FILL (used/capacity 80/90)
  + OVERSUB (Σ shared+enabled quotas / capacity, 2.0x) — independent. Σ from the ConfigJSON
  Descriptor (offsite.ReadDescriptor, new), never the report echo; dedicated+disabled
  excluded. Scope "pool-box" -> operator channel only, no SaveEvent. Failed fetch keeps the
  last snapshot degraded; missing data never becomes 0% and never transitions a band.
- config: Alerting.OffsiteBoxFill{Warn,Crit}Percent + OffsiteOversubWarnRatio (80/90/2.0
  defaults; thresholds pending Viktor's ruling). Constructed in the HETZNER_TOKEN branch,
  60s sweep, snapshot handed to the web server.
- web: Offsite-tab panel (fill bar, Σ+ratio, per-customer usage/quota rows) + a compact
  dashboard tile; reads the cached snapshot only, never fetches; nil -> "not configured".

Tests: 10 new + 4 red-proofs (throttle, Σ filter, escalation-only, failed-fetch honesty),
all confirmed red then restored. go build/vet/test all pass; hub confirm gate OK.

hub/internal/web/render_test.go

@@ -92,11 +92,15 @@ func TestTemplates_DashboardCriticalBadge(t *testing.T) {
 		EventErrors    int
 		EventWarnings  int
 	}
-	data := []dashboardCustomer{{
+	// v0.64.0: dashboard.html now takes {Customers, OffsiteTile}; OffsiteTile nil → no tile rendered.
+	data := struct {
+		Customers   []dashboardCustomer
+		OffsiteTile any
+	}{Customers: []dashboardCustomer{{
 		CustomerSummary: store.CustomerSummary{CustomerID: "c1", CustomerName: "Acme", ReceivedAt: time.Now()},
 		OverallStatus:   "ok", BackupAge: "–",
 		EventCriticals: 2, EventErrors: 1, EventWarnings: 0,
-	}}
+	}}}
 	var buf bytes.Buffer
 	if err := s.templates.ExecuteTemplate(&buf, "dashboard.html", data); err != nil {
 		t.Fatalf("render dashboard.html: %v", err)