admin: per-area permissions on every route

Give each user independent read, write and delete flags per area
(invoices, bingo, users), so someone can e.g. review and remove bingo
incidents without being able to add any. The request method and path
decide the flag needed: GET is read except the new/duplicate forms, POST
to .../delete is delete, any other POST is write. Unmapped admin paths
are superuser-only, and a test enumerates every registered admin route
so a new one cannot ship without a rule.

Pages follow the same flags: nav and dashboard tiles show readable areas
only, write forms and delete buttons are hidden, and the datacenter form
is shown read-only.

Assisted-by: Claude:opus-5

views/admin/dashboard.erb

@@ -4,6 +4,7 @@
 </div>
 
 <div class="dash">
+    <% if @inv %>
     <section class="section dash-tile">
         <h2><a href="/admin/invoices">Invoices</a></h2>
         <div class="stats">
@@ -28,9 +29,11 @@
                 </tbody>
             </table>
         <% end %>
-        <p><a href="/admin/invoices/new">+ New invoice</a> · <a href="/admin/invoices">all invoices</a></p>
+        <p><% if can?('invoices', 'write') %><a href="/admin/invoices/new">+ New invoice</a> · <% end %><a href="/admin/invoices">all invoices</a></p>
     </section>
+    <% end %>
 
+    <% if @bingo %>
     <section class="section dash-tile">
         <h2><a href="/admin/bingo">Bingo</a></h2>
         <div class="stats">
@@ -46,4 +49,8 @@
         <% end %>
         <p><a href="/admin/bingo">manage</a><% if bingo_public_url %> · <a href="<%= bingo_public_url %>" target="_blank" rel="noopener">public card ↗</a><% end %></p>
     </section>
+    <% end %>
+    <% unless @inv || @bingo %>
+        <section class="section"><p>No areas are assigned to your account yet. Ask an administrator for access.</p></section>
+    <% end %>
 </div>