hub v0.27.0: Hosts page — read-only fleet view (audit F-M1)

New Hosts nav section: a fleet list (/hosts) + per-host detail (/hosts/{id}),
read-only (GET only, no host actions). Surfaces identity, agent version,
online/stale status (reusing the HostStalenessChecker threshold), guests,
vitals, storage targets with SMART/thin-pool, and DR/escrow presence.

- store: new ListGuestsForHost reader (reality cols only; omits api_key/
  desired_spec_json) + scanGuest helper.
- web: handleHostsList + handleHostDetail (hosts.go); hosts.html +
  host_detail.html; Hosts nav link on every page; timeAgoPtr helper; routes.
- tests: store getter, both handlers, no-secret (api_key) assertion, 404,
  no-report empty state, status-band mapping.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hub/internal/web/templates/dashboard.html

@@ -15,6 +15,7 @@
                 <a href="/" class="nav-link active">Dashboard</a>
                 <a href="/configs" class="nav-link">Customers</a>
                 <a href="/apps" class="nav-link">Apps</a>
+                <a href="/hosts" class="nav-link">Hosts</a>
                 <a href="/configuration" class="nav-link">Configuration</a>
             </nav>
         </header>