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_settings.html

@@ -0,0 +1,48 @@
+{{define "head"}}
+<meta name="robots" content="noindex">
+{{end}}
+
+{{define "content"}}
+<h1>Nettstedsinnstillinger</h1>
+
+{{with .Data}}{{with .Settings}}
+<article>
+    <form method="POST" action="{{basePath}}/admin/settings">
+        <input type="hidden" name="csrf_token" value="{{$.CSRFToken}}">
+
+        <label for="site_name">
+            Nettstedsnavn
+            <input type="text" id="site_name" name="site_name"
+                   value="{{.SiteName}}" required>
+        </label>
+
+        <label for="site_description">
+            Beskrivelse
+            <textarea id="site_description" name="site_description"
+                      rows="3">{{.SiteDescription}}</textarea>
+        </label>
+
+        <fieldset>
+            <legend>Registreringsmodus</legend>
+            <label>
+                <input type="radio" name="signup_mode" value="open"
+                       {{if eq .SignupMode "open"}}checked{{end}}>
+                Åpen — alle kan registrere seg
+            </label>
+            <label>
+                <input type="radio" name="signup_mode" value="requests"
+                       {{if eq .SignupMode "requests"}}checked{{end}}>
+                Forespørsler — nye brukere må godkjennes
+            </label>
+            <label>
+                <input type="radio" name="signup_mode" value="closed"
+                       {{if eq .SignupMode "closed"}}checked{{end}}>
+                Stengt — ingen nye registreringer
+            </label>
+        </fieldset>
+
+        <button type="submit">Lagre innstillinger</button>
+    </form>
+</article>
+{{end}}{{end}}
+{{end}}