feat: add admin panel with user, tag, and signup management

Phase 4 — Admin Panel:
- Admin dashboard with user/fave/pending-request counts
- User management: create with temp password, reset password,
  enable/disable accounts (prevents self-disable)
- Tag management: rename and delete tags
- Signup request management: approve (creates user with
  must-reset-password) and reject pending requests
- Site settings: site name, description, signup mode
  (open/requests/closed)
- All admin routes require both login and admin role
- SignupRequest model and full store (create, list pending,
  approve with user creation, reject)
- SetMustResetPassword method on UserStore for admin password resets

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/pages/admin_dashboard.html

@@ -0,0 +1,41 @@
+{{define "head"}}
+<meta name="robots" content="noindex">
+{{end}}
+
+{{define "content"}}
+<h1>Administrasjon</h1>
+
+{{with .Data}}
+<div class="grid">
+    <article>
+        <header><strong>Brukere</strong></header>
+        <p class="stat">{{.UserCount}}</p>
+        <footer><a href="{{basePath}}/admin/users">Administrer brukere</a></footer>
+    </article>
+    <article>
+        <header><strong>Favoritter</strong></header>
+        <p class="stat">{{.FaveCount}}</p>
+    </article>
+    <article>
+        <header><strong>Ventende forespørsler</strong></header>
+        <p class="stat">{{.PendingCount}}</p>
+        {{if gt .PendingCount 0}}
+            <footer><a href="{{basePath}}/admin/signup-requests">Se forespørsler</a></footer>
+        {{end}}
+    </article>
+</div>
+
+<nav>
+    <ul>
+        <li><a href="{{basePath}}/admin/users" role="button" class="outline">Brukere</a></li>
+        <li><a href="{{basePath}}/admin/tags" role="button" class="outline">Merkelapper</a></li>
+        <li><a href="{{basePath}}/admin/signup-requests" role="button" class="outline">Forespørsler</a></li>
+        <li><a href="{{basePath}}/admin/settings" role="button" class="outline">Innstillinger</a></li>
+    </ul>
+</nav>
+
+{{with .Settings}}
+    <p><small>Registreringsmodus: <strong>{{.SignupMode}}</strong></small></p>
+{{end}}
+{{end}}
+{{end}}