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/templates/dashboard.html

@@ -22,7 +22,15 @@
             </nav>
         </header>
 
-        {{if not .}}
+        {{with .OffsiteTile}}
+        <a href="/offsite" class="offsite-tile offsite-tile-{{.Band}}">
+            <span class="offsite-tile-label">Offsite pool</span>
+            <span class="offsite-tile-val">{{formatFloat .FillPercent}}% &middot; {{.RatioStr}}</span>
+            {{if .Degraded}}<span class="offsite-tile-stale">stale</span>{{end}}
+        </a>
+        {{end}}
+
+        {{if not .Customers}}
         <div class="empty-state">
             <p>No customer reports received yet.</p>
             <p class="hint">Configure <code>hub.enabled: true</code> in customer controller.yaml to start receiving reports.</p>
@@ -44,7 +52,7 @@
                 </tr>
             </thead>
             <tbody>
-                {{range .}}
+                {{range .Customers}}
                 <tr class="status-{{.OverallStatus}}" onclick="window.location='/customers/{{.CustomerID}}'">
                     <td class="customer-name">
                         <span class="status-dot status-dot-{{statusColor .OverallStatus}}"></span>