fix(auth): add JSON tags to AccessSnapshot fields for proper serialization

ui/src/App.svelte

@@ -5,6 +5,7 @@
     key_id: string;
     last_accessed_at: string;
     last_path: string;
+    user_agent: string;
     request_count: number;
   };
 
@@ -224,6 +225,7 @@
                   <th class="px-4 py-3 font-medium">Principal</th>
                   <th class="px-4 py-3 font-medium">Last accessed</th>
                   <th class="px-4 py-3 font-medium">Last path</th>
+                  <th class="px-4 py-3 font-medium">Agent</th>
                   <th class="px-4 py-3 font-medium">Requests</th>
                 </tr>
               </thead>
@@ -242,6 +244,9 @@
                     <td class="px-4 py-3 align-top"
                       ><code class="text-slate-100">{entry.last_path}</code></td
                     >
+                    <td class="px-4 py-3 align-top text-slate-400 text-xs max-w-[16rem] truncate"
+                      >{entry.user_agent ?? "—"}</td
+                    >
                     <td class="px-4 py-3 align-top font-semibold text-white"
                       >{entry.request_count}</td
                     >