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/app.html

@@ -28,6 +28,10 @@
       <a href="/?lib=novel&amp;tab=all" class="{{if eq .Lib "novel"}}active{{end}}"
          {{if eq .Lib "novel"}}aria-current="page"{{end}}>Novels</a>
     </nav>
+    {{/* The owner's only difference on this page: a link out to the
+         administrative one. It sits beside Log out rather than in the library
+         switch — that switch says which library, not which page. */}}
+    {{if .Owner}}<a class="ghost" href="/admin">Admin</a>{{end}}
     <form method="post" action="/logout">
       <button type="submit" class="ghost">Log out</button>
     </form>
@@ -76,8 +80,6 @@
 
   {{template "setup" .}}
 
-  {{if .Owner}}{{template "readers" .}}{{end}}
-
   {{template "keyrow" .}}
 
   {{template "recent" .}}