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/style.css

@@ -298,6 +298,25 @@ header h1 {
     border-radius: var(--radius);
     transition: width 0.3s ease;
 }
+/* Fill bands (v0.64.0, R-5) — exception color: neutral when nominal, amber ≥ warn, red ≥ crit; no green. */
+.bar-fill.bar-ok { background: var(--text-3); }
+.bar-fill.bar-warning { background: var(--warn); }
+.bar-fill.bar-critical { background: var(--crit); }
+
+/* Offsite pool dashboard tile (v0.64.0, R-5) — compact, band-colored, links to /offsite. */
+.offsite-tile {
+    display: inline-flex; align-items: baseline; gap: 0.5rem;
+    padding: 0.4rem 0.8rem; margin-bottom: 1rem;
+    border: 1px solid var(--line); border-radius: var(--radius);
+    text-decoration: none; font-size: 0.85rem;
+}
+.offsite-tile-label { color: var(--text-3); text-transform: uppercase; font-size: 0.72rem; letter-spacing: 0.03em; }
+.offsite-tile-val { color: var(--text-1); font-weight: 600; }
+.offsite-tile-stale { color: var(--warn); font-size: 0.72rem; }
+.offsite-tile-warning { border-color: var(--warn); }
+.offsite-tile-warning .offsite-tile-val { color: var(--warn); }
+.offsite-tile-critical { border-color: var(--crit); }
+.offsite-tile-critical .offsite-tile-val { color: var(--crit); }
 
 /* Container table */
 .container-table {