web: owner-only /admin with the Reader roster and Poll Lane status (#102)

The only operational surface was /healthz and a fold-out roster inside the
owner's own reading page. This gives the owner a page: two sections of facts on
the same measured sheet, no cards.

- admin.go holds every route that reaches past the acting Reader, listed once
  in adminRoutes() and wrapped in requireOwner at registration - a missing gate
  is visible in the route list rather than hidden inside a handler. A non-owner
  gets 404, the same answer revoke already gave.
- Lane figures arrive through the LaneReporter seam, so the page reads the
  running poller rather than a table. newRouter converts a nil *Poller to a nil
  interface: a typed nil would make the page claim a poller exists.
- The roster moves out of the reading page and gains the Sighting counters, the
  blocked verdict and Clear marks. Clearing restores a privilege, so it is a
  plain ghost button; --danger stays with revocation.
- --patina is the page's one accent, held at the weight of the other action
  accents. Neither --ember (new chapter) nor --danger (destruction) is borrowed
  for system health.

backend/internal/web/templates/lanes.html

@@ -0,0 +1,36 @@
+{{/* Poll Lane status: one row per Site, refreshing itself so a run can be
+     watched rather than sampled by reloading. The refresh is one attribute on
+     the fragment root and the endpoint answers with this same fragment, so the
+     swap replaces the element that asked for it.
+
+     Every figure here is read out of the running poller, never out of a table:
+     a Site absent from Rows has not completed a pass since the last restart,
+     which the empty state must say — zeroes would read as a stopped Lane. */}}
+{{define "lanes"}}
+<section class="lanes" id="lanes" aria-live="polite"
+         hx-get="/ui/admin/lanes" hx-trigger="every 30s" hx-swap="outerHTML">
+  <h2>Poll Lanes</h2>
+  {{if .Rows}}
+  <ul class="lanelist">
+    {{range .Rows}}
+    <li{{if .Attention}} class="attention"{{end}}>
+      <span class="lane-site">{{.Site}}</span>
+      <span class="lane-fact">{{.Due}} due</span>
+      <span class="lane-fact">ran {{.Ran}}</span>
+      <span class="lane-fact">gap {{.Gap}}</span>
+      {{if .Clamped}}<span class="lane-mark">gap at floor</span>{{end}}
+      {{if .Refusing}}<span class="lane-mark">refusing</span>{{end}}
+      {{if .BrowserLost}}<span class="lane-mark">no browser</span>{{end}}
+    </li>
+    {{end}}
+  </ul>
+  {{else}}
+  <p class="setup-copy">No data yet — no Lane has completed a pass since the
+    backend started.</p>
+  {{end}}
+  <p class="setup-copy lane-browser">Browser sidecar:
+    {{if not .BrowserConfigured}}not configured — comix, kagane and novelfull
+    pages are not fetched through it{{else if .BrowserReachable}}reachable
+    {{else}}unreachable{{end}}.</p>
+</section>
+{{end}}