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), or for a
resubmitted copy the one the request it copies arrived at, and the
request headers as one block of text, one line per value, sorted by
name, whitespace kept. The event log leaves out headers that hold
more than the body's 32 KiB limit, stored or as text, and links to the
event's page, which shows them all. Both pages draw them through one
shared template, event_request.html, and read the webhook's
entrypoints 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>