hub v0.65.0 — PBS DR storage visibility (ep0 usage op) + Offsite tab split + dual dashboard gauges (R-5)

Makes PBS DR storage visible like the restic pool box (v0.64.0), differentiated. Scoping
correction: restic = subaccounts on the shared Hetzner Storage Box (Hetzner API); PBS DR =
the felhom-offsite PBS datastore on the ep0 endpoint VM (NO Hetzner API). Option A
(Viktor-ruled): a read-only `usage` op on the felhom-tenantsync ep0 forced command (twin of
fingerprint), polled by a new hub checker on the 15-min throttle. READ-ONLY throughout.

Phase-0 (gate PASSED): on ep0 (PBS 4.2.3), df -B1 --output=size,used,avail <datastore path>
yields bytes (39990112256/7627939840/... ~19%), read-only, existing sudo context, no admin token.

- scripts/felhom-tenantsync.sh -> v1.2.0: read-only `usage` short-circuit (df on the datastore
  path), no customer_id, no admin token, NO mutation. + a bash harness proving zero mutation.
- tenantsync.Client.Usage() + BoxUsage; unknown-op -> typed ErrUsageUnsupported (graceful).
- monitor.PBSDRBoxChecker: OffsiteBoxChecker clone over a usageReader seam; 15-min throttle,
  cached PBSBoxSnapshot, escalation-only pbsdr_box_fill on the "pbsdr-box" scope (operator only,
  no SaveEvent), recovery re-arm. Fill only. THREE states: ok / unavailable (ep0 <=v1.1.0,
  neutral no-alert) / degraded (exec failed, keep last).
- config: Alerting.PBSDRBoxFill{Warn,Crit}Percent (80/90); built with the tenantsync client,
  60s sweep, SetPBSDRBox. Hub deploy INDEPENDENT of the ep0 update (graceful degradation).
- web: /offsite splits into Restic + PBS DR hash tabs (endpoint cards under PBS DR); PBS panel;
  the single dashboard tile becomes two gauges (RESTIC pct.ratio, PBS DR pct / n/a).
- runbook offsite-endpoint.md 10: v1.2.0 update steps (no sudoers/authorized_keys change).

Tests: 10 Go + the harness; 3 red-proofs (usage mutation, escalation-only, unavailable-drives-band)
confirmed red then restored. go build/vet/test + bash -n + hub confirm gate all pass.

hub/internal/web/templates/dashboard.html

@@ -22,12 +22,23 @@
             </nav>
         </header>
 
-        {{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>
+        {{if or .OffsiteTile .PBSTile}}
+        <div class="offsite-gauges">
+            {{with .OffsiteTile}}
+            <a href="/offsite#tab=restic" class="offsite-tile offsite-tile-{{.Band}}">
+                <span class="offsite-tile-label">Restic</span>
+                <span class="offsite-tile-val">{{formatFloat .FillPercent}}% &middot; {{.RatioStr}}</span>
+                {{if .Degraded}}<span class="offsite-tile-stale">stale</span>{{end}}
+            </a>
+            {{end}}
+            {{with .PBSTile}}
+            <a href="/offsite#tab=pbsdr" class="offsite-tile offsite-tile-{{.Band}}">
+                <span class="offsite-tile-label">PBS DR</span>
+                {{if .Unavailable}}<span class="offsite-tile-val" style="color:var(--text-3);">n/a</span>{{else}}<span class="offsite-tile-val">{{formatFloat .FillPercent}}%</span>{{end}}
+                {{if .Degraded}}<span class="offsite-tile-stale">stale</span>{{end}}
+            </a>
+            {{end}}
+        </div>
         {{end}}
 
         {{if not .Customers}}