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/admin.html

@@ -0,0 +1,36 @@
+{{/* The owner's administrative page: everything that reaches past one Reader,
+     at its own address so it can be bookmarked rather than hunted for inside
+     the reading page. Owner-only at route registration (requireOwner), which
+     is why nothing in here re-tests who is asking. */}}
+{{define "admin"}}
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="color-scheme" content="dark light">
+  <title>BookmarkManager — Admin</title>
+  <link rel="icon" href="/static/logo.svg" type="image/svg+xml">
+  <link rel="stylesheet" href="/static/style.css">
+  <link rel="preload" href="/static/fonts/instrument-serif-400-latin.woff2" as="font" type="font/woff2" crossorigin>
+  <script src="/static/htmx.min.js" defer></script>
+</head>
+<body>
+<div class="sheet">
+  <header class="topbar">
+    <h1 class="brand">{{template "mark" .}}<span>Bookmark<em>Manager</em></span></h1>
+    {{/* Back to the library, no switch: this page belongs to neither library,
+         and the ember-lit switch says which library you are reading. */}}
+    <a class="ghost back" href="/">Library</a>
+    <form method="post" action="/logout">
+      <button type="submit" class="ghost">Log out</button>
+    </form>
+  </header>
+
+  {{template "lanes" .Lanes}}
+
+  {{template "readers" .}}
+</div>
+</body>
+</html>
+{{end}}