Show an event's entrypoint and request headers (closes #389)

An expanded event in the event log and the event's own page now show
the entrypoint the event arrived at (its description, "Entrypoint"
when it has none, or "deleted entrypoint"; never its URL) and the
full request headers, one per line, sorted by name. Both pages draw
them through one shared template, event_request.html. The event
columns now load entrypoint_id and headers, and the webhook's
entrypoints are read once per page.

Model: opus-5-5

templates/event_detail.html

@@ -50,6 +50,15 @@
         </dl>
     </div>
 
+    <div class="card mt-6">
+        <div class="p-4 border-b border-gray-200">
+            <h2 class="text-lg font-medium text-gray-900">Request</h2>
+        </div>
+        <div class="p-4">
+            {{template "event_request" .}}
+        </div>
+    </div>
+
     <div class="card mt-6">
         <div class="p-4 border-b border-gray-200">
             <h2 class="text-lg font-medium text-gray-900">Body</h2>